Embodiments of mechanisms for forming a semiconductor device are provided. The semiconductor device includes a semiconductor substrate and an isolation structure in the semiconductor substrate and surrounding an active region of the semiconductor substrate. The semiconductor device also includes a gate over the semiconductor substrate, and the gate has an intermediate portion over the active region and two end portions connected to the intermediate portion, and the end portions are over the isolation structure. The semiconductor device further includes a support film over the isolation structure and covering the isolation structure and at least one of the end portions of the gate. The support film exposes the active region and the intermediate portion of the gate.