Focusing on the system stability of large-scale photovoltaic （PV） plants connected to the grid, this paper establishes a complete electromechanical transient model of grid-connected PV plant on FASTEST （fast analysis of stability using the extended equal area criterion and simulation technologies, a power system transient security quantitative analysis software）. Based on FASTEST＇s unique transient security quantitative analysis, simulations are conducted for two typical cases： the PV plant was covered by cloud, and disturbance occurred on the system side. Studies are conducted to analyze the different impacts of the PV pant and the conventional power plant on the grid operation in terms of the system transient angle stability, transient voltage stability and transient frequency stability. The results show that, with the PV plant connected to the grid, the system transient frequency stability is most severely affected. Compared with a synchronous generator with excitation and governor system, the PV plant has a more remarkable transient response when a fault occurs on the grid side.