When etching monocrystalline silicon with UV laser, several action mechanisms codetermine the etching result, such as thermal conduction and plasma effect. Thermal conduction leads to material melting and evaporation, plasma effect leads to material breaking and throwing-out. Analyses theoretically the role of relevant process parameters in every action mechanism, tests respectively the key process parameters, ultimately obtains the relationship between the optimal process parameter and the best etching quality. On the basis of analysis and test, the guiding principle is obtained which is used to guide the experiment on how to get the best etching quality. The result shows the guiding principle is reasonable, and the optimal process parameter of etching monocrystalline silicon is achieved. The experiment shows, the power density of single pulse and frequency determines the slot’s depth and width, both power density of single pulse and scanning speed determine the shape and surface quality of the slot.