This paper presents a novel mechanical and magnetic model of the grid-connected photovoltaic system. Taking requirements and characteristics of present photovoltaic (PV) electromagnetic transient model into account, and taking the theory of power flow calculation as starting point, this paper approximately decouples the output power of the PV system, and uses the phase angle difference between the two ends of the filter and the modulation of the inverter to calculate the active and reactive power respectively, by which simulates the characteristics of the PV system in steady or transient processes. The model is developed in the commercial software package Power System Analysis Software Package (PSASP), and is compared with the PV model in PSASP, considering the disturbance of illumination and three-phase grounding fault in the grid, to validate the accuracy of the model. The simulated results demonstrate that the proposed model is correct, efficient and well encapsulated, which is suitable for the transient stability analysis of a network containing a grid-connected PV model and provides reference for the research of large scale grid-connected PV generation system.