Nepenthes thorelii is a shrub with an erect stem having a smooth circular cross section and a tapering form.
The stem reaches 40 cm in height and is 4 to 8 mm in diameter.
The species has a well-developed perennial rootstock that produces shoots every wet season.
The rootstock is irregularly branched and measures up to 2 cm in width.

Leaves vary in shape from linear-lanceolate to narrowly obovate.
The lamina or leaf blade measures up to 26 cm in length by 3 cm in width.
Its apex is acute to acuminate.
The base of the lamina is amplexicaul and decurrent into two wings that extend up to 2.5 cm down the stem.
The ends of these wings are rounded such that they almost touch.
Two to four longitudinal veins are present on either side of the midrib.
Pinnate veins are numerous and curve towards the apex of the lamina.

Rosette and lower pitchers are ovoid in shape and measure up to 11.5 cm in height by 4.5 cm in width.
They are light green with reddish markings and a reddish lid.
A pair of wings up to 8 mm wide runs down the front of each pitcher.
These wings bear fringe elements up to 5 mm long and spaced around 2 mm apart.
The pitcher mouth is ovate-triangular and has an oblique insertion.
The peristome is rounded at the front and ranges in width from 2 to 4 mm at the front to 7 mm at the rear.
It bears a series of ribs spaced 0.25 to 0.4 mm apart.
The inner margin of the peristome is lined with rounded teeth measuring 0.2 to 0.5 mm.
The lid or operculum is ovate to rounded in shape and up to 3.5 cm long by 2.8 cm wide.
Numerous prominently lipped glands are present on the underside of the lid, particularly near the base of the midrib, where they are 0.3 to 0.7 mm wide.
Glands become less dense and smaller (around 0.15 mm wide) towards the margins.
An unbranched spur measuring 2 to 4 mm is inserted near the base of the lid.

Upper pitchers arise from uncurled tendrils.
They are obovate, becoming narrower towards the mouth.
They are similar in size to their lower counterparts, reaching 12.5 cm in height by 4.5 cm in width.
In aerial pitchers, the broad wings of the lower pitchers are reduced to narrow structures only 1 to 1.5 mm wide, with shorter acuminate fringe elements (≤1.5 mm long) spaced 3 to 7 mm apart.
As in lower pitchers, the pitcher mouth is oblique and concave.
The peristome is rounded and 3 to 5 mm wide, with a regularly undulate outer margin.
The lid is similar to that of the lower pitchers.

Nepenthes thorelii has a large racemose inflorescence.
The peduncle is 8 to 18 cm long, while the rachis is 50 to 70 cm long.
Partial peduncles are one-flowered.
Pedicels are up to 6 mm long and may or may not be shortly bracteate.

The species has an indumentum of simple or branched white hairs between 0.3 and 0.4 mm long.

The diploid chromosome number of N. thorelii is 78 according to a 1969 paper by Katsuhiko Kondo.
However, a 1997 study by Günther Rudolf Heubl and Andreas Wistuba found the chromosome number to be 80, and this is now accepted as the diploid number of all species in the genus.
Nepenthes thorelii is a shrub with an erect stem having a smooth circular cross section and a tapering form. The stem reaches 40 cm in height and is 4 to 8 mm in diameter. The species has a well-developed perennial rootstock that produces shoots every wet season. The rootstock is irregularly branched and measures up to 2 cm in width.
Leaves vary in shape from linear-lanceolate to narrowly obovate. The lamina or leaf blade measures up to 26 cm in length by 3 cm in width. Its apex is acute to acuminate. The base of the lamina is amplexicaul and decurrent into two wings that extend up to 2.5 cm down the stem. The ends of these wings are rounded such that they almost touch. Two to four longitudinal veins are present on either side of the midrib. Pinnate veins are numerous and curve towards the apex of the lamina.
Rosette and lower pitchers are ovoid in shape and measure up to 11.5 cm in height by 4.5 cm in width. They are light green with reddish markings and a reddish lid. A pair of wings up to 8 mm wide runs down the front of each pitcher. These wings bear fringe elements up to 5 mm long and spaced around 2 mm apart. The pitcher mouth is ovate-triangular and has an oblique insertion. The peristome is rounded at the front and ranges in width from 2 to 4 mm at the front to 7 mm at the rear. It bears a series of ribs spaced 0.25 to 0.4 mm apart. The inner margin of the peristome is lined with rounded teeth measuring 0.2 to 0.5 mm. The lid or operculum is ovate to rounded in shape and up to 3.5 cm long by 2.8 cm wide. Numerous prominently lipped glands are present on the underside of the lid, particularly near the base of the midrib, where they are 0.3 to 0.7 mm wide. Glands become less dense and smaller (around 0.15 mm wide) towards the margins. An unbranched spur measuring 2 to 4 mm is inserted near the base of the lid.
Upper pitchers arise from uncurled tendrils. They are obovate, becoming narrower towards the mouth. They are similar in size to their lower counterparts, reaching 12.5 cm in height by 4.5 cm in width. In aerial pitchers, the broad wings of the lower pitchers are reduced to narrow structures only 1 to 1.5 mm wide, with shorter acuminate fringe elements (≤1.5 mm long) spaced 3 to 7 mm apart. As in lower pitchers, the pitcher mouth is oblique and concave. The peristome is rounded and 3 to 5 mm wide, with a regularly undulate outer margin. The lid is similar to that of the lower pitchers.
Nepenthes thorelii has a large racemose inflorescence. The peduncle is 8 to 18 cm long, while the rachis is 50 to 70 cm long. Partial peduncles are one-flowered. Pedicels are up to 6 mm long and may or may not be shortly bracteate.
The species has an indumentum of simple or branched white hairs between 0.3 and 0.4 mm long.
The diploid chromosome number of N. thorelii is 78 according to a 1969 paper by Katsuhiko Kondo. However, a 1997 study by Günther Rudolf Heubl and Andreas Wistuba found the chromosome number to be 80, and this is now accepted as the diploid number of all species in the genus.
Nepenthes thorelii