This paper summarizes the causes of commutation failure and points out that two main causes of commutation failure are AC system ground fault and fire pulse missing. The mathematic model of the junction temperature of thyristor valve is collated and the influence of the single-phase ground fault, the two-phase ground fault, the three-phase ground fault and the fire pulse missing of the converter on the junction temperature are simulated respectively. And it is concluded that the fire pulse missing that has the greatest influence on the junction temperature. Based on the consideration of those influences, it’s suggested that the temperature characteristic of the valve needs to be taken into account when the commutation failure protection settings are fixed.