The invention relates to an organic semiconductor thin film transistor, a fabrication method thereof, and a display device and a backplate which are provided with the organic semiconductor thin film transistor. The organic semiconductor thin film transistor comprises a source, a drain and a grid, wherein a channel layer is arranged on the source and the drain, a channel is formed in the channel layer, used for making at least part of the source and the drain exposed and filled with an organic semiconductor layer, the organic semiconductor layer is connected with the source and the drain, and a pattern is formed on the organic semiconductor layer. The depth and the extension direction of the channel are easily controlled to be endowed with high consistency during the fabrication process, and thus, in different components applying the organic semiconductor thin film transistor in such structure, the thickness of the organic semiconductor layer is more uniform; and moreover, organic semiconductor materials are more easily arranged along a channel direction, namely, the consistency of the arrangement direction of the organic semiconductor materials is also better, so that the organic semiconductor thin film transistors in difference components have relatively uniform characteristics.