Embodiments of the present invention disclose a manufacturing method of a thin film transistor, a thin film transistor, an array substrate and a display device. The manufacturing method of a thin film transistor includes a step of forming an active layer, and the step of forming an active layer includes: forming a first poly-silicon layer and a second poly-silicon layer on the first poly-silicon layer separately, and adding dopant ions into the second poly-silicon layer and an upper surface layer of the first poly-silicon layer. By using the manufacturing method of a thin film transistor, defect states and unstable factors of interface in the thin film transistor can be reduced, thereby improving stability of the LTPS thin film transistor and obtaining an array substrate and a display device having more stable performance.