Line commutated converter based high voltage direct current (LCC-HVDC) links are widely employed for long distance bulk power transmission and asynchronous alternating current (AC) grid connection. However, LCC-HVDC systems often suffer from commutation failures when AC voltage is distorted, oscillating or reduced by AC faults, which leads to overheating of converter valves and interruptions in transmitted power. Seriously, it would have adverse impact on the safety and stability of the entire power system. This paper proposes a supplementary control for mitigation of successive commutation failures on the basis of analyzing the influence of phase-locked loop (PLL) dynamics on the commutation process. By analyzing the impact of PLL dynamics on the actual leading angle, it is found that changes in the AC voltage phase remarkably influence commutation. Accordingly, the error between AC voltage phase and PLL's output angle is added to the output of the extinction angle or DC voltage control to mitigate the successive commutation failures of LCC-HVDC. Simulations conducted on the CIGRE benchmark model in PSCAD/EMTDC validate the performance of the supplementary control, which effectively mitigates successive commutation failures.