In order to study the new subsynchronous oscillation problem caused by the interaction between the controller and the grid in the direct drive wind farm through the AC grid-connected system, comprehensively considering the internal control dynamic characteristics, AC dynamic response and power transmission characteristics of the converter, the dq equivalent impedance model of the direct-drive wind turbine through the AC grid-connected system is deduced. Combined with the stability criterion of the system impedance, the influence of the change of the internal control parameters of the grid-side converter of the direct-drive wind turbine on the subsynchronous oscillation is analyzed. Research shows that as the PI control parameters (Kp, Ki) of the inner and outer loops of the grid-side converter decrease, the risk of oscillation instability increases and the system stability decreases. Finally, the correctness of the model and theoretical analysis is verified by time-domain simulation under PSCAD/EMTDC environment.