A method for forming a concave bottom oxide layer in a trench, comprising: providing a semiconductor substrate; forming a pad oxide layer on the semiconductor substrate; forming a silicon nitride layer on the pad oxide layer; etching the silicon nitride layer, the pad oxide layer and the semiconductor substrate to form the trench in the semiconductor substrate; depositing a silicon oxide layer to refill into the trench and cover on the silicon nitride layer, wherein the silicon oxide layer has overhang portions at corners of the trench; anisotropically etching the silicon oxide layer to form a concave bottom oxide layer in the trench; etching the silicon oxide layer to remove the silicon oxide layer on the silicon nitride layer and the sidewalls of the trench; removing the silicon nitride layer and the pad oxide layer.