The invention discloses manufacturing methods of a thin film transistor and an array substrate, the array substrate and a display device. The manufacturing method of the thin film transistor comprises the steps of forming a first photoresist pattern including two photoresists with different thicknesses on a pattern of a semiconductor layer; performing a heavily-doped ion implantation process on the pattern of the semiconductor layer by using the first photoresist pattern as a barrier mask to form patterns of a source heavily-doped region and a drain heavily-doped region; ashing the first photoresist pattern to remove the photoresist with the second thickness, thin the photoresist with the first thickness and form a second photoresist pattern; and performing a lightly-doped ion implantation process on the pattern of the semiconductor layer by using the second photoresist pattern as a barrier mask to form patterns of a channel region, a source lightly-doped region and a drain lightly-doped region. The method can accurately and efficiency control the key sizes of the source lightly-doped region and the drain lightly-doped region, and reduces the steps so as to shorten the production time, reduce the cost and improve the product quality.