Abstract: The oscillation problem of renewable energy generation connected to power grid system is an important research topic worthy of attention. The primary research task is to reveal the mechanism of oscillation and research the key influencing factors. In this paper, a small-signal mathematical model of photovoltaic(PV) grid-connected system equipped with static var generator (SVG) is built firstly. Then, the characteristic oscillation mode and the dynamic links strongly related to it of PV grid-connected system are studied by eigenvalue analysis, and the influence of AC system on the oscillation characteristics of PV grid-connected system is analyzed. The mechanism of subsynchronous oscillation (SSO) is explained from the point of view of circuit resonance, and the electro-magnetic simulations on the PSCAD/EMTDC is carried out to verify its correctness. The research results indicate that there is a dominant SSO mode in the PV grid-connected system, and the control parameters of the converter have important influences on the stability of this mode. There is a control interaction between SVG and PV, which requires reasonable design of control parameters. The research can provide guidance for the operation and control design of photovoltaic grid-connected systems.