With the development of photovoltaic power generation, largescale photovoltaic plants gridconnection has a significant impact on power system stability. A damping control introduced in photovoltaic gridconnected inverters helps to increase the damping of the power system. However, too large a damping control parameter causes a new oscillation mode. Under this background, a smallsignal model is established for photovoltaic plants connected to a singlemachine infinite bus system to analyze the influence of photovoltaic plants gridconnection on system damping and design active power additional damping controller for the photovoltaic gridconnection. Furthermore, the contribution of the additional damping controller is theoretically analyzed to the system damping, and the system root locus is utilized to clarify the mechanism of oversize additional damping control coefficient affecting the system stability. And then, the computation method of the control parameters is proposed when the system is at the maximum damping. Finally, the correctness of the theoretical analysis is verified by the simulation on PSCAD software.