A sliding mode active disturbance rejection control (SM-ADRC) strategy based on the current inner loop of the grid-side converter is proposed to address the problem of sub-synchronous oscillation (SSO) resulting from the amplification of the SSO components. First, considering the issue of high-frequency switching in sliding mode control (SMC) that affects the control performance, an improved exponential reaching law is designed, by replacing the original sign function with a boundary variable power function in the reaching law. Then, an extended state observer with active disturbance rejection control is introduced to estimate the SSO components in real-time and compensate for the current inner loop as observed quantities. This weakens the dependence of SM-ADRC control on the parameters of the grid-side controller and further cuts off the propagation path of the SSO components. Finally, an electromagnetic transient model of the direct-drive wind farm based on grid-side current inner loop SM-ADRC control is established. By comparing the SSO suppression effect with the current inner loop PI control and SMC control, the superiority of the proposed SM-ADRC strategy is verified.