The invention relates to an organic semiconductor thin film transistor, a fabrication method thereof, a display device with the same and a back plate. The organic semiconductor thin film transistor comprises a source, a drain, a grid and a channel layer, wherein a channel is formed in the channel layer, an organic semiconductor layer is arranged in the channel layer, a pattern is formed on the organic semiconductor layer, and the source and the drain are arranged on the channel layer and connected with the organic semiconductor layer. Since the depth and the extension direction of the channel are easily controlled during fabrication, high consistence is achieved, therefore, in different components of the organic semiconductor thin film transistor applying such structure, the thickness of the organic semiconductor layer is more uniform, moreover, the organic semiconductor material is easy to be arranged along a channel direction, the consistence of the organic semiconductor material arrangement direction is high, and the organic semiconductor thin film transistors in the different components have uniform characteristic.