A method for forming a semiconductor device structure is provided. The method includes forming a first layer over a substrate. The first layer has a trench. The method includes forming first spacers over inner walls of the trench. The method includes removing a portion of the first spacers. The method includes forming a filling layer into the trench to cover the first spacers. The filling layer and the first spacers together form a strip structure. The method includes removing the first layer. The method includes forming second spacers over two opposite first sidewalls of the strip structure.