Abstract: To improve the frequency and voltage regulation ability of large capacity photovoltaic plant,the applications of virtual synchronous generators（VSG） technology had aroused wide concern.The analysis on whether VSG would cause stability problem is becoming an important issue to ensure the safety of power system.Therefore,a small-signal model of grid-connected photovoltaic VSG is developed.The potential oscillation modes are analyzed based on the calculation of eigenvalue.We focus on the characters of new mode caused by the application of VSG.The influence of the VSG control and system parameters on oscillation modes are researched based on the small-signal analysis and controller hardware in loop simulation.The design method VSG control parameter is then proposed.The result of this paper shows that VSG may have an appreciable impact on the system small-signal stability and security constraints of smallsignal stability should be considered when designing the control parameters.