The invention provides a transistor, a preparation method thereof, a display substrate and a display device. The transistor comprises a substrate structure, a gate electrode, a source electrode, a drain electrode and an active layer, wherein the gate electrode, the source electrode, the drain electrode and the active layer are arranged on the substrate structure. The gate electrode, the source electrode and the drain electrode are all located at one side, deviating from the substrate structure, of the active layer. The active layer comprises a first region corresponding to an orthographic projection region of the gate electrode on the substrate structure and a second region located outside the orthographic projection region, wherein the second region is made of a conductive semiconductor material, and the surface, in contact with the first region, of the substrate structure and the surface, in contact with the second region, of the substrate structure are not in the same plane. The surface, in contact with the first region, of the substrate structure and the surface, in contact with the second region, of the substrate structure are not in the same plane, so the transistor enables the height difference to be formed between the first region and the second region of the active layer, and the diffusion path of an active layer material from the second region to the first region in the conductor formation is lengthened, and the diffusion of the active layer material needs greater energy, thereby improving the poor threshold voltage uniformity of the transistor caused by the short-channel effect.