The present invention further provides a method for forming a semiconductor device, comprising: first, a substrate having a fin structure disposed thereon is provided, wherein the fin structure has a trench, next, a first liner in the trench is formed, a first insulating layer is formed on the first liner, afterwards, a shallow trench isolation is formed in the substrate and surrounding the fin structure, wherein a bottom surface of the shallow trench isolation is higher than a bottom surface of the first insulating layer, and a top surface of the shallow trench isolation is lower than a top surface of the first insulating layer, and a dummy gate structure is formed on the first insulating layer and disposed above the trench, wherein a bottom surface of the dummy gate structure and a top surface of the fin structure are on a same level.