A method for forming a semiconductor device structure is provided. The method includes performing a first plasma etching process on a substrate to form a first trench in the substrate. The method includes removing a second portion of the substrate under the bottom surface to form a second trench under and connected to the first trench. The second trench surrounds a third portion of the substrate under the first portion. The third portion has a first sidewall. The first sidewall is inclined relative to the top surface at a second angle, and the first angle is greater than the second angle. The method includes forming an isolation structure in the first trench and the second trench. The method includes forming a gate insulating layer over the top surface and the first inclined surface. The method includes forming a gate over the gate insulating layer and the isolation structure.