A method for forming a semiconductor device includes providing a to-be-etched layer, forming a first mask layer on the to-be-etched layer, forming a patterned core layer on the first mask layer of a first region, forming a sidewall spacer material layer on the core layer and the first mask layer, removing the sidewall spacer material layer on a top surface of the core layer, removing the core layer and the first mask layer at a bottom of the core layer to form a first trench, removing the sidewall spacer material layer on the first mask layer of a second region, forming a first patterned layer exposing the first mask layer of the second region, and using the first patterned layer as a mask to remove the first mask layer of the second region to form a second trench.