This paper proposes a unified method for inter-area power oscillations which is the key to solve coordination problems caused by different mathematical models and control theories. Firstly, the unified model, breaking the constraints of the network topology, component and the precise extents in the process, is established based on global signals. Secondly, making use of the structural feature of the unified system model, it combines advanced control theory to resolve the problem of parameter uncertainty and model error. Finally, the control law is effectively derived with backstepping method. The simulation results in Matlab show the validity of the proposed method.