A thin film transistor, a manufacturing method thereof, an array substrate, and a display device are provided. The thin film transistor comprises a base substrate, a gate on the base substrate, a gate insulating layer covering the gate, an active layer on the gate insulating layer, a first electrode and a second electrode over and electrically connected to the active layer, and a first insulating portion between the gate insulating layer and the first electrode. An orthographic projection of the first insulating portion on the base substrate, an orthographic projection of the first electrode on the base substrate, and an orthographic projection of a boundary between a side surface of the gate and an upper surface of the gate on the base substrate at least partially overlap.