The present invention provides a LTPS array substrate and a manufacturing method thereof. The method comprises: forming a source electrode and a drain electrode on a substrate, forming polysilicon layers of a first region and a second region on the substrate including the source electrode and the drain electrode, and the thickness of the polysilicon layer of the first region is greater than the one of the second region, the polysilicon layer of the first region partially covers the source electrode and the drain electrode; passivating the surface of the polysilicon layer in order to turn the part of the adjacent surface of the polysilicon layer of the second region and the first region into an insulating layer; forming a gate electrode on the insulating layer between the source electrode and the drain electrode. The present invention can simplify the LTPS technical process and reduce the producing costs.