The inverter connected to the grid employs a phase-locked loop to synchronize with the grid, and its dynamic characteristics can impact the stability of the system. Moreover, due to the resistance and inductance of the grid in the weak grid, the control loop of the grid-connected inverter interacts with the grid impedance, making the system stability analysis more complicated. Thus, an output admittance model for the LCL grid-connected inverter has been developed in the d-q frame, which considers the phase-locked loop dynamics, decoupling terms and delay terms. This study investigates the relationship between the resonance characteristics of the output admittance and the filter parameters. In addition, the effects of grid strength, PLL bandwidth and current loop bandwidth on stability are also comprehensively analyzed. The simulation results indicate that an increase in PLL bandwidth, current loop bandwidth, and grid impedance leads to a decrease in the stability of the grid-connected system.