For the subsynchronous oscillation (SSO) of a multi-machine system in a long-distance and large-capacity transmission system, the suppression strategy using the additional current of STATCOM is analyzed. Modal decoupling control method is adopted, and the parameter setting process of the controller is decribed in detail. The port equivalent admittance matrix of each component in the network is established. The establishment of the complex frequency domain network equation of the whole system can be simplified by eliminating variables dispersedly in complex torque coefficient analysis, and it is convenient to analyze the equivalent electrical damping characteristics of the generator. Finally, the SSO characteristics of an actual series-compensated transmission system and the suppression effect of STATCOM on SSO problems are analyzed by this method. The results show the device can increase the electrical damping of all unstable modes and suppress subsynchronous oscillation effectively. This provides a reference for solving the SSO problems of multi-machine and multi-mode in the systems. This work is supported by the National Key Research and Development Program of China (No. 2017YFB0902002).