In the hybrid direct current (DC) transmission system composed of modular multi-level converter (MMC) and line commutated converter (LCC),the commutation failure of LCC seriously affects the security and stability of the system. Firstly,the current characteristics and alternating current (AC)/DC voltage characteristics of MMC-LCC hybrid DC transmission system are analyzed when commutation failure occurs. Secondly,considering the influence of modulation ratio on half-bridge MMC,the feasible region of voltage modulation ratio is extended by using enhanced control strategy of MMC voltage. Then,the segmented control strategy of MMC voltage is proposed. According to the different degrees of AC voltage sag,the adjustment methods of DC voltage reference value are designed respectively,and the voltage control logic of hybrid HVDC transmission system is optimized to realize the effective switching of MMC voltage between normal operation and fault condition. Finally,the model of MMC-LCC hybrid DC transmission system is built in MATLAB/Simulink,and the simulation results show that the proposed control strategy can improve the DC voltage control accuracy and system stability while achieving fault ride through.