The hybrid HVDC system, which adopts line commutated converter in rectifier side, and modular multilevel converter in inverter side, can combine the advantages of the two converters and has broad application prospects. Based on the study of the basic steady-state operation control characteristics, the DC transmission power drop and the interruption problem caused by the asymmetric fault of the AC power network are analyzed emphatically on the basis of the LCC-MMC hybrid HVDC transmission system. And the AC fault characteristics of the hybrid HVDC system are analyzed. It is found that the DC voltage drop caused by the asymmetrical fault is most likely to cause the interruption of the transmission power at the rectifier side, and cause the voltage anomaly at the inverter side. A supplementary DC voltage control strategy based on MMC typical control is proposed to improve the transmission capacity of the DC system by reducing the reference DC voltage of the inverter in the modulation range. If it is detected that the inverter side DC component of the DC voltage exceeds the limit value, the additional control strategy is automatically input, without relying on the communication between the converter stations. Finally, the feasibility of the proposed control strategy is verified by PSCAD/EMTDC electromagnetic transient simulation.