Xixiasaurus is estimated to have been around 1.5 metres (5 ft) long, and to have weighed about 8 kilograms (18 lb).
Since the nasal bones of the holotype specimen were not fused, it may not have been a mature individual.
As a troodontid maniraptoran, it would have been bird-like, lightly built, with raptorial (grasping) hands and an enlarged sickle-shaped claw on the hyper-extendible second toe.
Fossils of other troodontids, such as Jianianhualong, show that members of the group were covered in pennaceous feathers, with long feathers on the arms and legs, and frond-like feathering on the tail (similar to the avialan Archaeopteryx).

The skull of Xixiasaurus was long, similar to that of Byronosaurus overall, and the snout was distinctly long and low.
The premaxillae were not fused to each other, and bore a few small pits on their side surfaces.
When viewed from below, the front margin of the upper jaw formed a tapering U-shape, distinct from the shape in other troodontids; the U-shape tapered less in Byronosaurus and was very elongated in Sinornithoides, for example.
Viewed from above, the nasal processes that extended hindwards from the premaxillae were wedged between the premaxillary processes of the nasal bones, which extended forwards.
The nasal process of the premaxilla ended at the level of the hind margin of the external narial opening (bony nostril), and was quadrangular in cross-section rather than triangular, as in Byronosaurus.
The nasal process of the premaxilla formed the upper hind corner of the narial opening, and Xixiasaurus was distinct in having an opening on the side surface at the base of the nasal process.
The suture between the premaxilla and maxilla curved upwards from the front of the snout, and straightened out under the narial opening when seen from the side.
The maxillary process of the premaxilla tapered hindwards and wedged between a small forwards-extended process of the maxilla and the main part of that bone.
The maxilla was kept from being part of the margin of the narial opening by the maxillary process.
The maxillary process of the premaxilla extended hindwards to the same level as the nasal process.
Due to not being fused together, the premaxillae had a fissure along their lower midline.
There were two small openings (connected to each other by a shallow groove) near the base of the third and fourth premaxillary teeth, but none near the first and second.

The maxilla was much longer than high, forming most of the skull's side surface, and to the front, most of the lower margin of the narial opening.
The narial opening was roughly oval, with its long axis (the longest stretch from its front to the back) being parallel with the lower margin of the maxilla.
The side surface of the maxilla below the narial opening was covered with small pits and foramina (openings), as in Byronosaurus, and the row of these was just above and parallel to the margin of the tooth-row.
The side surface of the maxilla had three large, elongated openings, called the anterior, maxillary, and antorbital fenestrae.
The anterior fenestra appears to have been entirely enclosed by the maxilla, and there were two rows of small pits below it.
The back of the maxillary fenestra had a bony wall called the interfenestral bar, which separated it from the antorbital fenestra, as in Byronosaurus.
The antorbital fenestra (the largest of the three openings, located in front of the orbit) was rectangular in side view, and the part of the maxilla below it was low and did not have small foramina, unlike the front part.
The maxillary teeth were placed along most of the lower margin of the antorbital fenestra.
On the lower side, the maxilla formed an extensive internal shelf which contributed to a large secondary palate (which separated the nasal airways from the mouth), extending backwards from the contact with the premaxilla.
The front part of the palatal shelf had three small openings, and there was a row of nutrient foramina (which allowed blood to supply the bone with nutrients) in a groove just above and parallel to the tooth row
Xixiasaurus is estimated to have been around 1.5 metres (5 ft) long, and to have weighed about 8 kilograms (18 lb). Since the nasal bones of the holotype specimen were not fused, it may not have been a mature individual. As a troodontid maniraptoran, it would have been bird-like, lightly built, with raptorial (grasping) hands and an enlarged sickle-shaped claw on the hyper-extendible second toe. Fossils of other troodontids, such as Jianianhualong, show that members of the group were covered in pennaceous feathers, with long feathers on the arms and legs, and frond-like feathering on the tail (similar to the avialan Archaeopteryx).
The skull of Xixiasaurus was long, similar to that of Byronosaurus overall, and the snout was distinctly long and low. The premaxillae were not fused to each other, and bore a few small pits on their side surfaces. When viewed from below, the front margin of the upper jaw formed a tapering U-shape, distinct from the shape in other troodontids; the U-shape tapered less in Byronosaurus and was very elongated in Sinornithoides, for example. Viewed from above, the nasal processes that extended hindwards from the premaxillae were wedged between the premaxillary processes of the nasal bones, which extended forwards. The nasal process of the premaxilla ended at the level of the hind margin of the external narial opening (bony nostril), and was quadrangular in cross-section rather than triangular, as in Byronosaurus. The nasal process of the premaxilla formed the upper hind corner of the narial opening, and Xixiasaurus was distinct in having an opening on the side surface at the base of the nasal process. The suture between the premaxilla and maxilla curved upwards from the front of the snout, and straightened out under the narial opening when seen from the side. The maxillary process of the premaxilla tapered hindwards and wedged between a small forwards-extended process of the maxilla and the main part of that bone. The maxilla was kept from being part of the margin of the narial opening by the maxillary process. The maxillary process of the premaxilla extended hindwards to the same level as the nasal process. Due to not being fused together, the premaxillae had a fissure along their lower midline. There were two small openings (connected to each other by a shallow groove) near the base of the third and fourth premaxillary teeth, but none near the first and second.
The maxilla was much longer than high, forming most of the skull's side surface, and to the front, most of the lower margin of the narial opening. The narial opening was roughly oval, with its long axis (the longest stretch from its front to the back) being parallel with the lower margin of the maxilla. The side surface of the maxilla below the narial opening was covered with small pits and foramina (openings), as in Byronosaurus, and the row of these was just above and parallel to the margin of the tooth-row. The side surface of the maxilla had three large, elongated openings, called the anterior, maxillary, and antorbital fenestrae. The anterior fenestra appears to have been entirely enclosed by the maxilla, and there were two rows of small pits below it. The back of the maxillary fenestra had a bony wall called the interfenestral bar, which separated it from the antorbital fenestra, as in Byronosaurus. The antorbital fenestra (the largest of the three openings, located in front of the orbit) was rectangular in side view, and the part of the maxilla below it was low and did not have small foramina, unlike the front part. The maxillary teeth were placed along most of the lower margin of the antorbital fenestra. On the lower side, the maxilla formed an extensive internal shelf which contributed to a large secondary palate (which separated the nasal airways from the mouth), extending backwards from the contact with the premaxilla. The front part of the palatal shelf had three small openings, and there was a row of nutrient foramina (which allowed blood to supply the bone with nutrients) in a groove just above and parallel to the tooth row 
Xixiasaurus