The design of unified controller for effectively damping the inter-area power oscillations is proposed. This new controller can solve the coordination problems among the different mathematical models and control theories. During the modeling process， a unified interconnected power system model is established by breaking the constraints of the network topology， the fine degree of component model and the adaptability range. The backstepping method is introduced due to the structural feature of the unified power system model， and it combines the advanced control theories to solve the parameter uncertainty and model error problems in the control law deriving process. The validity of the design is verified by the relevant simulation results in MATLAB.