The invention provides a method for determining the height of an insulating material in a shallow trench. The method comprises the following steps: providing shallow trench isolation structures with different insulating material heights; simulating mechanical pressure stress of the insulating material in the shallow trench generated to a semiconductor substrate when the insulating material is in different heights; and selecting the height of the corresponding insulating material in the shallow trench as an optimal value when the mechanical pressure stress is minimum. The method selects the optimal height of the insulating material to ensure the optimal depression degree on the edge of the shallow trench and minimum generated leakage current.