Aiming at the problem of commutation failure in high voltage direct current (HVDC) transmission system,the unified power flow controller (UPFC) is applied to reduce the probability of commutation failure of HVDC system,and improve the original control strategy of the unified power flow controller.When the AC side faults,UPFC shunt control strategy using AC bus voltage reference value adjustment method to suppress commutation failure.The severity of fault is introduced into the fault level FL to characterize the HVDC system,through theoretical analysis and simulation verification of PSCAD/EMTDC:When the fault level of F_L=18.7%,which can control the original control strategy and improvement strategy of UPFC inhibition after commutation failure;fault level F_L=23.1%,only the improved control strategy can effectively inhibit the occurrence of commutation failure;failure is more serious,when the fault level reached F_L=93.56%,the improved control strategy can effectively reduce the probability of occurrence of continuous commutation failure.Reactive power compensation can be affected by AC system strength enhanced HVDC system using UPFC dynamic,improve the commutation failure resistance,thereby effectively improving the operation performance of HVDC system.