The green tree python is characterized by a relatively slim body. The relatively long tail accounts for about 14% of the total length. The head is large and clearly defined from the neck. The snout is large and angular. The body is triangular in cross section with a visible spine. The species usually reaches a total length of 150–180 cm (4.9–5.9 ft), but large females may reach 200 cm (6.6 ft). The size also varies depending on the region of origin. The weight is highly dependent upon the nutritional status of the animal. Males can weigh about 1,100–1,400 g (2.4–3.1 lb), females up to 1,600 g (3.5 lb), although wild specimens are typically much lighter than this. Especially large specimens that can weigh up to 2,200 g (4.9 lb) are invariably females, which like most snakes are slightly larger and heavier than males.
Primarily arboreal, these snakes have a particular way of resting in the branches of trees; they loop a coil or two over the branches in a saddle position and place their head in the middle.
The diet consists mostly of small mammals, such as rodents, and sometimes reptiles. This snake, like the emerald tree boa, was thought to eat birds; however, Switak conducted field work on this issue. In examining stomach contents of more than 1,000 animals, he did not find any evidence of avian prey. Prey is captured by holding onto a branch using the prehensile tail and striking out from an s-shaped position and constricting the prey. Wild specimens have also been observed and photographed wrapped around the base of small tree trunks, facing down in an ambush position, presumably waiting for ground mammals to prey upon.