A semiconductor device, and a manufacturing method thereof. The method includes: providing a semiconductor substrate provided with a body region, a gate dielectric layer, and a field oxide layer, formed on the semiconductor substrate; forming a gate polycrystalline, the gate polycrystalline covering the gate dielectric layer and the field oxide layer and exposing at least one portion of the field oxide layer; forming a drift region in the semiconductor substrate by ion implantation using a drift region masking layer as a mask, removing the exposed portion of the field oxide layer by further using the drift region masking layer as the mask to form a first field oxide self-aligned with the gate polycrystalline; forming a source region in the body region, and forming a drain region in the drift region; forming a second field oxide on the semiconductor substrate; and forming a second field plate on the second field oxide.