To address the issue of subsynchronous oscillation (SSO) arising when photovoltaic virtual synchronous generators (VSG_PV) transmit power through modular multi-level converter-based high-voltage direct current (MMC-HVDC) systems, a linearized mathematical model of the system is developed. An improved virtual synchronous control strategy is proposed. Using the eigenvalue analysis method, the study reveals that, under power disturbances, several factors-including the active and reactive control links, virtual inductance, MMC-HVDC bridge arm inductance, and current vector control loop-significantly affect the damping and frequency characteristics of SSO in the VSG-PV system. These findings are validated through simulation on the PSCAD/EMTDC platform. Results show that the integration of the VSG function introduces a sub-synchronous oscillation mode in which both the photovoltaic VSG and MMC-HVDC participate during power disturbances in the outgoing system. On the VSG side, an excessively large active frequency modulation coefficient K_f or a too-small virtual inductance L_v can lead to system instability; on the MMC-HVDC side, increasing the integral coefficient of the voltage loop K_i4 may induce system instability, whereas increasing the bridge arm inductance L_g enhances system stability.