A method for forming a semiconductor device structure includes forming a first dummy gate stack and a second dummy gate stack over a semiconductor substrate and forming a dielectric layer over the semiconductor substrate to surround the first dummy gate stack and the second dummy gate stack. The method includes removing the first dummy gate stack and the second dummy gate stack to form a first trench and a second trench in the dielectric layer and removing the first dummy gate stack and the second dummy gate stack to form a first trench and a second trench in the dielectric layer. The method includes partially removing the first metal gate stack, the second metal gate stack, and the dielectric layer to form a recess. The method includes forming an insulating structure to partially or completely fill the recess.