The present invention provides a method for improving the quality of a high-voltage metal oxide semiconductor (HV MOS), the method includes: firstly, a substrate is provided, next, a hard mask layer is formed on the substrate, an oxygen plasma treatment is then performed to the hard mask layer, so as to form an oxide layer on the hard mask layer. Afterwards, a patterned photoresist layer is formed on the oxide layer, and a first cleaning process is performed to a top surface of the oxide layer after the patterned photoresist layer is formed, wherein the first cleaning process comprises rinsing the oxide layer with carbonated water. Next, a first etching process is performed to remove parts of the hard mask layer, and the patterned photoresist layer is then removed. Afterwards, a second etching process is performed, to remove the oxide layer.