The transient process caused by commutation failure in AC-DC interconnected system is not the same as that in pure AC system, which has effect on the performance of distance protection. This paper takes single-phase earth fault as an example to analyze the effect of commutation failure on ground distance relay based on double fault sources model. Analysis results show that the calculated short circuit reactance is not accurate by using Fourier algorithm during commutation failure. Then, AC risk factor is defined and its expression is also provided. Then, the countermeasures are put forward based on AC risk factor and protection algorithm. At last, a large amount of simulation results using PSCAD/EMTDC prove the conclusion to be reasonable and the countermeasures are compared. The comparison results show that differential equation algorithm has better performance in commutation failure environment.