Subsynchronous oscillation(SSO) shows the characteristics of nonlinear oscillation due to the nonlinearity of power system. But most of the existing SSO controllers are designed by linear method, and the effect is imperfect. The order of the system model is too large because SSO has multiple oscillation modes, which makes it difficult to use the nonlinear method to design the controller. So it is necessary to reduce the order of the system model before using the nonlinear control method. In this paper, the nonlinear sliding mode variable structure control method, which has strong robustness, is applied to the design of SSO damping controller. Firstly, the modal perturbation order reduction method is used to reduce the order of the SSO system model, which preserves the key state variables. Then the parameters of SVC damping controller are tuned by pole assignment which is carried out using variable structure control. Finally compared with the traditional phase compensation method, simulation results are given to verify this proposed method can effectively suppress multiple SSO modes at the same time.