Abstract: As MMC-HVDC system is of a large number of applications, the mechanism of its influence on the SSO characteristics of the adjacent units needs to be studied. This article proposed the coupling loop of the voltage disturbance between HVDC system and the AC bus, and further established the response characteristics equations of MMC-HVDC control system between amplitude and phase perturbation of the AC bus voltage. Then, the damping characteristics based on response characteristic equations above were analyzed to reveal the phenomena and reason of the weak connection between MMC-HVDC and the units’ SSO characteristics. Finally, the results are verified by electromagnetic transient simulations. The results show that MMC-HVDC system has weaker damping effect on the units’ SSO characteristics.