Abstract: With continuous increase in penetration rate of photovoltaic (PV) generation, PV generation systems are required to possess grid-forming capability. To meet current limiting requirements, existing methods generally adopt control architectures with cascaded power outer loop, voltage loop and current loop, or introduce virtual admittance links in control loop. Impedance modeling and grid-connected stability of virtual synchronous generator (VSG)-based grid-forming PV inverters incorporating virtual admittance and current inner control loops were investigated. Firstly, the impedance model of the PV inverter under VSG control was established and validated. Secondly, the influence laws of key factors such as the power outer loop parameters, the parameters and structures of virtual admittance link and the cutoff frequency of low-pass filters on the impedance characteristics of the PV inverter were analyzed. Subsequently, based on the generalized Nyquist stability criterion and the system loop impedance method, the influence laws of control parameters and grid strength on the small-signal stability of VSG-based grid-forming PV inverters were investigated. Finally, the accuracy of stability conclusions was verified through MATLAB/Simulink simulations. The results demonstrate that a smaller virtual inductance value facilitates stable operation in medium-low frequency bands; appropriately increasing both virtual resistance in virtual admittance link and cutoff frequency of low-pass filter can effectively enhance system stability margin; under strong grid conditions, the grid-forming PV inverter systems may encounter low-frequency oscillation instability risks.