Commutation failure is one of the most common faults in inverter sides of DC systems， mainly because of the AC system faults. This paper builds the commutation failure risk index system and further proposes the risk evaluation approach of multi-infeed HVDC system under AC system fault conditions considering multiple uncertain factors. The proposed approach uses the commutation voltage-time area as criterion. The case of China Southern Power Grid 2009 operation mode demonstrates that the commutation failure risk evaluation is of significant practical and engineering values to explore the weakness of commutation failure in the HVDC transmission system. It can guide planning and operation of power grid， prevent and mitigate the commutation failures， and help maintaining secure and stable operation of power systems.