The invention relates to a method for forming a P-type light doping drain electrode region by the pre-noncrystallization processing of germanium. The invention provides a method for forming a MOS device with an ultra shallow light doping diffusion area. The method comprises the following steps of: providing a semiconductor substrate with a surface area; providing a grid dielectric layer covering the surface area and forming a grid electrode covering one part of the grid dielectric layer; and by using the grid electrode structure as a mask, carrying out the first injection process by germanium matters so as to form a noncrystallization area in the light doping drain electrode region on the semiconductor substrate.