A method for forming a semiconductor device is provided. The method includes forming an isolation structure in a semiconductor substrate. The method includes forming a gate over the semiconductor substrate. The method includes forming a support film over the isolation structure. The support film is a continuous film which continuously covers the isolation structure and the gate over the isolation structure, the support film conformally covers a first portion of a top surface and a second portion of a first sidewall of the gate, the top surface faces away from the semiconductor substrate, the support film and a topmost surface of the active region do not overlap with each other, and the topmost surface faces the gate. The method includes after forming the support film, forming lightly doped regions in the semiconductor substrate and at two opposite sides of the gate.