The present disclosure relates to a COA substrate and a manufacturing method thereof, a display device. The COA substrate includes the following layers in a wiring region: a first conduction layer including a signal wire lead; an insulation layer having a first via to expose the signal wire lead; a second conduction layer including a first metal pattern with a second via; a flat layer having a hollowed-out region; and a connection pattern. The second via is at least in part located within the hollowed-out region, and orthographic projections of a border of a portion of the second via which is located within the hollowed-out region and a border of the first via on the base substrate overlap. The connection pattern extends through the first via and the second via and connects to the signal wire lead.