Aiming at the problem of subsynchronous oscillation(SSO) when Photovoltaic virtual synchronous generators (VSG_PV) are sent out of the system through modular multi-level converter based on high voltage direct current (MMC-HVDC), a linearized mathematical model of the system is established. This paper proposes an improved virtual synchronous control strategy. The eigenvalue method is used to analyze that when the power is disturbed, the active control link, reactive control link, virtual inductance, MMC-HVDC bridge arm inductance, and current vector control link of the photovoltaic virtual synchronous power generation system will all affect the damping characteristics and frequency characteristics of the system SSO, and it is verified on the PSCAD/EMTDC simulation platform. The results show that, with the introduction of VSG function, there is a sub-synchronous oscillation mode in which photovoltaic VSG and MMC-HVDC participate in the process of power disturbance in the outgoing system. On the photovoltaic VSG side, the active frequency modulation coefficient Kf is too large or the virtual inductance Lv is too small, which will lead to system instability; On the MMC-HVDC side, the increase of voltage loop integral coefficient Ki4 will lead to system instability, while the increase of bridge arm inductance Lg will enhance system stability.