When connected to the grid, sub-synchronous oscillations may occur in the wind farms composed of Doubly-Fed Induction Generators (DFIG). This paper builds the system model of the DFIG-based wind farms that are connected to the grid via series compensation capacitors, and then analyses the main factors that affect the characteristics of the sub-synchronous oscillations with the Nyquist stability criterion. On this basis, a new control strategy based on stator-side virtual resistance is proposed to suppress the sub-synchronous oscillation by enhancing the electrical damping within the sub-synchronous frequency band. The simulation results in time domain show that this method can effectively suppress the occurrence of sub-synchronous oscillation and improve the stability of the system. Compared with the traditional suppressing method, the new strategy in this paper does not need any additional devices, thus it is favorable for engineering applications. This work is supported by National Natural Science Foundation of China (No. 51707096) and Open Fund Project of Jiangsu Wind Power Engineering Technology Center (No. ZK16-03-02).