Abstract: Aiming at the problem that the inertial synchronous control strategy and its sub-synchronous oscillation characteristics are rarely studied for single-stage grid-forming photovoltaic power generation systems, this paper applies inertial synchronous control to gridforming photovoltaic power generation systems, establishes its state-space model, and uses eigenvalue analysis to determine the oscillation mode of the system. Finally, the influence of system parameters on the oscillation mode is analyzed by using the participation factor and eigenvalue root locus. The research results show that grid-forming photovoltaic system has better adaptability to weak power grid, but in strong power network, there is SSO mode dominated by DC capacitance and inertia synchronization, which can increase the system damping and improve the system stability by increasing the DC capacitance and voltage inner loop proportional coefficient or reducing the voltage inner loop integral coefficient. Based on the PSCAD/EMTDC time domain simulation platform, the correctness of the theoretical analysis is verified.