A COA substrate manufacturing method including: forming a TFT on a base substrate; forming a second insulation layer on the TFT; forming a color resist layer on the second insulation layer; forming a third insulation layer on the color resist layer; forming a through hole which reveals the drain electrode of the TFT; forming an ITO film layer on the third insulation layer; forming a photoresist layer on the ITO film layer; performing a light-shielding process to the photoresist layer on the vias-region ITO film layer and an exposure process to the photoresist layer on the non vias-region ITO film layer; developing the photoresist layer on the vias-region ITO and the non vias-region ITO film layers to obtain a photoresist layer plug covered on the vias-region ITO film layer. The present invention utilizes the photoresist to fill the through hole which can improve the quality of a display device.