Abstract: Optical lithography is widely used in the micro-electro-mechanical system (MEMS) and integrated circuit. As the lithographic process equipment is very expensive, it is critical to utilize optical lithography simulation to predict the useful results and optimize process problems. In this paper, the waveguide (WG) method based on rigorous electromagnetic field model is presented. It is firstly extended to simulate the thick photoresist exposure in MEMS field. By applying this model, the light intensity distribution in the photoresist can be simulated. Thus, the morphology of photoresist after development can be predicted. Some examples demonstrate the validity of this model.