Abstract: The integration of modular multilevel converter based high voltage direct current（MMC-HVDC） can cause lowfrequency oscillation（LFO） instability between synchronous generators（SGs）. Therefore, this paper studies the mechanism of influence of MMC-HVDC with power synchronization control（PSC） and vector current control（VCC） on the damping torque of SGs. First, the modified Heffron-Philips dynamic model of SG considering the influence of modular multilevel converter（MMC） is built. Then, by using the complex torque analysis method, the mechanism that leads to negative damping torque of generators induced by MMC through network dynamic coupling is studied. Furthermore, under the above two control strategies, the critical control links of MMC that can lead to LFO between SGs are discussed and compared through parameter sensitivity analysis.Finally, an auxiliary control strategy is proposed for the PSC based MMC to convert the damping torque of SGs caused by its integration from negative to positive. Based on the electromagnetic transient simulation model of a multi-machine system built in PSCAD/EMTDC, the effectiveness and robustness of the proposed auxiliary control strategy are verified.