Abstract: When the mono-polar blocking fault occurs in line-commutated-converter based high voltage direct current (LCC-HVDC) transmission system, the filter in the converter station will be cut off in delayed groups, resulting in persistent overvoltage in the new energy station of the wind-photovoltaic-hydro hybrid sending end. In order to make hydropower units participate in restraining the overvoltage of sending end system caused by DC mono-polar blocking, an additional excitation control with AC bus voltage of HVDC converter station as input signal was proposed as the coordinated control strategy of hydropower units. When DC mono-polar blocking occurred, the advance phase depth of hydropower units was determined based on the overvoltage amplitude of converter station, and the surplus reactive power of the system was absorbed dynamically, so as to improve the overvoltage condition of the new energy station. Then, based on the output limit of excitation system and voltage stability limit of generator terminal, the setting rules of additional excitation controller gain upper limit in coordination control strategy were proposed. Finally, Taking Qinghai power grid as the research background, the effectiveness of coordinated control strategy was verified by PSCAD/EMTDC simulation. Simulated results indicate that the strategy proposed in this paper effectively exploits the ability of hydropower units to participate in the emergency voltage regulation of the system, and fills the short-term vacancy of reactive power regulation in sending end system caused by filter cutting delay. The method can effectively restrain the system overvoltage, which has engineering guiding significance.