A method for forming a semiconductor device structure is provided. The method includes forming a first gate stack structure and a second gate stack structure on a substrate, and the first gate stack structure includes a first spacer adjacent to the second gate stack structure. The method also includes forming an U-shaped capping layer between the first gate stack structure and the second gate stack structure, and a lateral sidewall of the U-shaped capping layer is in direct contact with the first spacer of the first gate stack structure. A top of the lateral sidewall of the U-shaped capping layer is below a top of the first spacer of the first gate stack structure.