The invention provides a film transistor and a manufacturing method thereof, and an array substrate and a display device. The film transistor comprises an active layer, an insulation layer arranged on the active layer, a first electrode layer and a second electrode layer, wherein the insulation layer comprises first through holes and a second through hole, the first through holes comprise a lower through hole close to the active layer and an upper through hole connected with the lower through hole, the smallest inner diameter of the upper through hole is greater than or equal to the largest inner diameter of the lower through hole, the first electrode layer is arranged in the upper through hole of the insulation layer, the first electrode is connected with the active layer through the lower through hole, and the second electrode layer is arranged on the insulation layer and is connected with the active layer through the second through hole. Through the film transistor and the manufacturing method thereof, and the array substrate and the display device, the product yield is improved.