This paper analyzes the mechanism of commutation failure and points out that the voltage sag of commutation bus in inverter station caused by AC system fault is a main factor. Different load models will lead to different amplitude of voltage sag, which will influence the occurrence of commutation failure. This paper analyses the impact on system voltage sag of two common used load models —— ZIP load and motor load, and points out that compared with ZIP model, the motor load model is more beneficial for preventing voltage sag at the instant of fault as the motor load can provide mechanical inertia to the system. While in the duration of fault, the motor load will absorb large amount of reactive power, which will do harm to the system stability. Based on the data of China Southern Power Grid (CSG), this paper calculates the fault regions that can cause commutation failure and the system critical clearance time under different load models to verify the view point proposed.