A method for forming a semiconductor device structure is provided. The semiconductor device structure includes a plurality of first nanostructures stacked over a substrate in a vertical direction. The semiconductor device structure includes a first bottom layer formed adjacent to the first nanostructures, and a first insulating layer formed over the first bottom layer. The semiconductor device structure includes a first source/drain (S/D) structure formed over the first insulating layer, and the first insulating layer is in direct contact with one of the first nanostructures.