The interaction between the direct drive permanent magnet synchronous generator (D-PMSG) and the grid is paid great attention in weak grid connection condition. At present, impedance-based analysis method is the main research means for the SSO issues of the system. The influence of DC voltage perturbation and DC voltage loop are ignored in the traditional impedance modeling process based on dq frame, and the influence of the control parameters of the wind turbine on the stability of the system are not analyzed. Therefore, a direct drive permanent magnet synchronous generator-based wind farm connected to an AC grid is selected as an example, the impedance of D-PMSG based on dq frame is established, and the dynamic characteristics of the PLL, DC voltage perturbation and DC voltage loop are taken into account, and the correctness of the impedance model is verified by the simulation "scanning". Then, the influence factors of the system stability are analyzed based on the determinant of the total impedance matrix of the system. The foundation for the optimization of the control parameters of the wind turbine are proposed. Finally, the correctness and effectiveness of the theoretical analysis are verified by the simulation results in time domain simulation.