Subsynchronous oscillation threatens the security and stability of power grid connected with wind farms. Subsynchronous oscillation suppression of grid-connected wind farms is one of the most important and practical issues in the study of the stability of grid-connected clean energy. FACTS /VSC-HVDC and its additional damping controller, subsynchronous damping controller and subsynchronous filiter of the control loop on the converter of wind power generator, and other measures are four types of main measures to suppress subsynchronous oscillation of grid-connected wind farms. This paper reviews and compares the research findings of these four types of suppression measures in the last ten years. The mechanism and designing steps of subsynchronous damping controller to suppress subsynchronous oscillation are also described in detail. Finally, the existing problems in the study of the subsynchronous oscillation suppression measures of the grid-connected wind farms are summarized. The paper also points out the direction for future research.