Trimeresurus stejnegeri grows to a maximum total length of 75 centimetres (30 in), which includes a tail length of 14.5 centimetres (5.7 in).
The males have hemipenes that are short and spinose beyond the bifurcation.

The dorsal scales are arranged in 21 longitudinal rows at midbody.
There are 9–11 upper labials, of which the first are separated from nasal scales by a distinct suture.
The supraoculars are single, narrow, and sometimes divided by a transverse suture.
There are 11–16 scales in a line between the supraoculars.
The ventrals number 150–174, and the subcaudals are 54–77.
All of the subcaudals are paired.

The color pattern is bright to dark green above, pale green to whitish below, the two separated by a bright bicolored orange or brown (below) and white (above) (males) or bicolored or white only (females) ventrolateral stripe, which occupies the whole of the outermost scale row and a portion of the second row.
Trimeresurus stejnegeri grows to a maximum total length of 75 centimetres (30 in), which includes a tail length of 14.5 centimetres (5.7 in). The males have hemipenes that are short and spinose beyond the bifurcation.
The dorsal scales are arranged in 21 longitudinal rows at midbody. There are 9–11 upper labials, of which the first are separated from nasal scales by a distinct suture. The supraoculars are single, narrow, and sometimes divided by a transverse suture. There are 11–16 scales in a line between the supraoculars. The ventrals number 150–174, and the subcaudals are 54–77. All of the subcaudals are paired.
The color pattern is bright to dark green above, pale green to whitish below, the two separated by a bright bicolored orange or brown (below) and white (above) (males) or bicolored or white only (females) ventrolateral stripe, which occupies the whole of the outermost scale row and a portion of the second row.
Trimeresurus stejnegeri