The invention provides an III nitride-based double-heterojunction phototransistor, belongs to the technical field of semiconductor devices and provides an III nitride-based double-heterojunction phototransistor and a preparation method thereof. The III nitride-based double-heterojunction phototransistor comprises a substrate and an epitaxial layer, wherein the epitaxial layer grows on the substrate; and the epitaxial layer sequentially comprises a buffer layer or a transition layer, a donor heavily-doped ohmic contact layer, an alloy composition gradient layer, a donor doped layer of a relatively broad band gap material, an unintentionally doped layer of the relatively broad band gap material, an acceptor doped layer, an unintentionally doped light absorption layer, the alloy composition gradient layer and a donor heavily-doped window layer of the relatively broad band gap material from bottom to top. By an overhead collector region structure, with the III nitride multi-element alloy material with a relatively broad band gap light absorption layer as the window layer of an incident light, the quantum efficiency is improved; and the number of photo-generated holes is increased, so that the photocurrent gain of the device is improved. A reverse heterojunction is adopted as an emitter junction in a lower emitter region; and the unintentionally doped layer of the relatively broad band gap material is introduced as an acceptor-doped diffusion barrier layer and a base region-emitter region heterointerface energy band sag compensation layer, so that the electron injection efficiency of the transistor is improved. The III nitride-based double-heterojunction phototransistor has the characteristics of high photocurrent gain, stable device performance and the like.