Abstract: Line commutated converter based high voltage direct current（LCC-HVDC） transmission system is prone to commutation failure with a fault in AC power grid, which has a great impact on the safe and stable operation of power system.Commutation failure prevention control（CFPREV） based on rapid detection of AC voltage drop is the main measure to avoid the first commutation failure. In this paper, the influences of the fault initial time, dynamic changes of DC current and responses of extinction angle controller on the commutation process are analyzed. From the two aspects of the critical commutation voltage causing the first commutation failure and the firing angle adjustment considering the fault severity,an improved strategy of CFPREV for HVDC transmission systems is proposed, which can enhance the immunity of HVDC against the first commutation failure and reduce the probability of HVDC commutation failure.