A thin film transistor (TFT), a manufacturing method thereof, a display substrate and a display device are disclosed. The TFT includes: a gate electrode; a gate insulating layer disposed on the gate electrode; a first active layer disposed on the gate insulating layer; a second active layer disposed on the first active layer, having a length smaller than that of the second active layer; a source electrode disposed on the first active layer, being contacted with a first side of the second active layer; and a drain electrode disposed on the first active layer, being contacted with a second side of the second active layer. Embodiments of the present invention can increase an ON-state current and meanwhile reduce an OFF leakage current in the TFT.