Abstract: When a failure occurred in the AC system at the inverter side of the line commutated converter-based high voltage direct current (abbr. LCC-HVDC) power transmission grid, then the commutation failure is easy to occur. For this reason, the commutation failure prevention (abbr. CFPREV) is the main measure to avoid the commutation failure. To solve the problem that the given CFPREV threshold could not suit to all fault conditions, starting from the principle of commutation failure, the relation between commutation failure and the two factors, i.e., the CFPREV start threshold and the regulating quantity gain of trigger angle was analyzed, similarly the influence of AC busbar voltage on CFPREV start threshold and the regulating quantity gain of trigger angle as well as the influence of the time of failure occurrence on the CFPREV start threshold were analyzed. It was proposed that taking AC busbar voltage as indicator the CFPREV start threshold and the gain were optimized, so the immunity for the commutation failure was improved to a certain degree. The proposed method is verified by the simulation on PSCAD/EMTDC platform, and simulation results show that the comparing with conventional CFPREV start threshold the optimized CFPREV start threshold can suppress the occurrence of commutation failure more effectively.