A method for forming a semiconductor device is provided. The method includes forming an isolation structure in a semiconductor substrate, and the isolation structure surrounds an active region of the semiconductor substrate. The method also includes forming a gate over the semiconductor substrate, and the gate is across the active region and extends onto the isolation structure. The gate has an intermediate portion over the active region and two end portions connected to the intermediate portion, the end portions are over the isolation structure. The method includes forming a support film over the isolation structure, and the support film is a continuous film which continuously covers the isolation structure and at least one end portion of the gate.