For doubly fed induction generator (DFIG)-based wind farms, the occurrence of sub-synchronous oscillation (SSO) is detrimental to the system operation and should be suppressed promptly. A low-cost control strategy based on reactive power regulation of DFIG-based wind farm is proposed to suppress SSO. First, the influence of the reactive power output of wind farms on SSO is studied in the entire operating region of DFIGs, and several uncertainty factors are taken into account. Furthermore, a reactive power regulation-based control strategy is proposed to suppress SSO by changing the operating point of the system. In addition, a PASTd (Projection Approximation Subspace Tracking based on the Deflation Technique)-based identification method is utilised to realise the online SSO monitoring. The implementation of the proposed control strategy is low cost since it does not require any additional supplementary equipment. The time-domain simulation results have also verified the effectiveness and strong robustness of the proposed control strategy.