Abstract: To mitigate the sub synchronous oscillation (SSO) in large-scale photovoltaic (PV) plants integrated to weak AC networks, a static var generator (SVG) based supplementary damping control strategy is proposed. The strategy extracts the voltage at the PV coupling point as input signal, and produces the sub-synchronous current that is in the same phase with the sub-synchronous voltage at the coupling point through supplementary sub-synchronous damping controller (SSDC). Thus, the SVG is equivalent to the positive resistance of the accessing the system in parallel at the sub-synchronous frequency, and dissipates the SSO energy for suppression. In addition, the structure and parameter design method of SSDC are analyzed in detail. Finally, taking a PV plant in northwest China as an example, the mitigation effectiveness of the proposed control strategy is verified under various working conditions through electromagnetic transient simulation, which shows that the proposed control strategy can improve the stability margin of the photovoltaic system.