Lü and colleagues assigned Xixiasaurus to the family Troodontidae based on its high tooth-count, constriction between the crowns and roots of the teeth, close packing of teeth near the tip of the dentary, and distinct groove for the neurovascular foramina on the dentary.
They found Xixiasaurus to be most closely related to Byronosaurus of Mongolia, and suggested the two may have formed a clade with Urbacodon from Uzbekistan consisting of troodontids with unserrated teeth, which radiated across Asia (while noting that serrations had been lost independently in different groups of theropods).
A  2012 phylogenetic analysis by the paleontologist Alan H. Turner and colleagues instead found Xixiasaurus to belong in a clade with Sinovenator and Mei (both also from China), due to sharing a maxillary process of the premaxilla that separated the maxilla from the nasal behind the narial opening.
In 2016, the palaeontologists Alexander Averianov and Hans-Dieter Sues did not identify a clade formed of troodontids with unserrated teeth, but found them to be successive sister taxa to a more derived (or "advanced") clade of troodontids with serrated teeth.
They suggested that the D-shaped cross-section of the premaxillary teeth could be one possible feature uniting Xixiasaurus, Byronosaurus, and Urbacodon.

The following cladogram shows the position of Xixiasaurus within Troodontidae according to a 2017 analysis by the palaeontologist Caizhi Shen and colleagues:
In 2019, the palaeontologist Scott Hartman and colleagues recovered Xixiasaurus as the sister taxon of Sinusonasus, in a clade with Daliansaurus and Hesperornithoides (sharing features such as a straight ulna and having an upwards projected curve on the claw of the first finger).

Troodontids have mainly been discovered in the northern hemisphere, largely restricted to Asia and North America.
They appear to have reached their greatest diversity during the Campanian stage of the Late Cretaceous in Asia.
Troodontids probably originated in Asia, and if genera such as Anchiornis are considered part of the family, it would have evolved by the Early Jurassic.
Troodontids are grouped in the clade Paraves, along with dromaeosaurids and birds.
Troodontids and dromaeosaurids have also been grouped together in the clade Deinonychosauria, based on the shared sickle-claw on their hyper-extendible second toe, but some studies have found troodontids to be more closely related to birds than to dromaeosaurids.
Lü and colleagues assigned Xixiasaurus to the family Troodontidae based on its high tooth-count, constriction between the crowns and roots of the teeth, close packing of teeth near the tip of the dentary, and distinct groove for the neurovascular foramina on the dentary. They found Xixiasaurus to be most closely related to Byronosaurus of Mongolia, and suggested the two may have formed a clade with Urbacodon from Uzbekistan consisting of troodontids with unserrated teeth, which radiated across Asia (while noting that serrations had been lost independently in different groups of theropods). A  2012 phylogenetic analysis by the paleontologist Alan H. Turner and colleagues instead found Xixiasaurus to belong in a clade with Sinovenator and Mei (both also from China), due to sharing a maxillary process of the premaxilla that separated the maxilla from the nasal behind the narial opening. In 2016, the palaeontologists Alexander Averianov and Hans-Dieter Sues did not identify a clade formed of troodontids with unserrated teeth, but found them to be successive sister taxa to a more derived (or "advanced") clade of troodontids with serrated teeth. They suggested that the D-shaped cross-section of the premaxillary teeth could be one possible feature uniting Xixiasaurus, Byronosaurus, and Urbacodon.
The following cladogram shows the position of Xixiasaurus within Troodontidae according to a 2017 analysis by the palaeontologist Caizhi Shen and colleagues:
In 2019, the palaeontologist Scott Hartman and colleagues recovered Xixiasaurus as the sister taxon of Sinusonasus, in a clade with Daliansaurus and Hesperornithoides (sharing features such as a straight ulna and having an upwards projected curve on the claw of the first finger).
Troodontids have mainly been discovered in the northern hemisphere, largely restricted to Asia and North America. They appear to have reached their greatest diversity during the Campanian stage of the Late Cretaceous in Asia. Troodontids probably originated in Asia, and if genera such as Anchiornis are considered part of the family, it would have evolved by the Early Jurassic. Troodontids are grouped in the clade Paraves, along with dromaeosaurids and birds. Troodontids and dromaeosaurids have also been grouped together in the clade Deinonychosauria, based on the shared sickle-claw on their hyper-extendible second toe, but some studies have found troodontids to be more closely related to birds than to dromaeosaurids.
Xixiasaurus