Semiconductor devices and fabrication methods thereof are provided. An exemplary fabrication method includes providing a base substrate; forming an isolation layer in the base substrate; forming dummy gate structures on the base substrate at two sides of the isolation layer; forming an additional gate structure on the isolation layer and a first protective layer on surfaces of the additional gate structure and the dummy gate structures; forming an interlayer dielectric layer covering side surfaces of the dummy gate structures, the additional gate structure and the first protective layer over the base substrate; removing a portion of the first protective layer over the additional gate structure; forming a second protective layer on the additional gate structure; removing portions of the first protective layer over the dummy gate structures using the second protective layer as a mask; and removing the dummy gate structures to form openings in the interlayer dielectric layer.