Abstract: Quickly and accurately assessing the risk of commutation failure in multi-infeed（high voltage direct current,HVDC）systems when faults occurring at the receivingend AC system, have great significance to ensure the safe and stable operation of large power grids. Inspired by the definition of multi-infeed interaction factor（MIIF）, the concept of ACDC voltage coupling factor（ADVCF） was proposed. Based on the node impedance matrix, a simple calculation formula of ADVCF was derived. The calculation formula of the critical AC-DC voltage coupling factor（CADVCF） was derived by the minimum extinction angle criteria. A method based on CADVCF to fast evaluate commutation failure risk of multiinfeed HVDC system was proposed: when a three-phase shortcircuit fault is occurring at an receiving-end system AC bus, if the ADVCF between one DC system and the AC bus is larger than the CADVCF between them, commutation failure will occur in the DC system. The simulation results of the actual large-scale system prove the speediness and accuracy of the proposed assessment method.