A method for automatically forming a sub-resolution PSM is provided. The shielding layer is designed by adding an assist feature to a peripheral region of an original shielding layer formed on a quartz substrate. Using an etching process with a etching mask, a portion of the original shielding layer is removed to form an original pattern and an assist feature. The assist feature is separated from the original pattern by a distance. A photoresist layer is tormed on the rim of the shielding layer so that the original pattern, half of the assist feature, and an exposed portion of the quartz substrate between the original pattern and the assist feature are exposed. A selective etching process is performed to etch the exposed portion of the quartz substrate to a certain depth so that it behaves like a phase shifting layer. After removing the photoresist layer, the sub-resolution PSM including the integrated circuit pattern and the assist feature is complete.