The present disclosure provides a TFT array substrate and a manufacturing method thereof. For the manufacturing method, a source electrode and a drain electrode are formed at first, and then edges of the source electrode and the drain electrode are used as masks to pattern a semiconductor layer to form an amorphous silicon island, which makes edges of the amorphous silicon island flush with the edges of the source electrode and the drain electrode, and completely removes the exposed semiconductor layer outside a metal layer, thereby decreasing photoelectric sensitivity of a TFT device, decreasing a size of the TFT device, and being beneficial for saving layouts and simplifying processes.