The power electronic trend of power system is more and more obvious, which brings more challenges to the application of traditional Lyapunov direct method in transient analysis of power system. Fortunately, the Lyapunov function method based on Sum-Of-Squares optimization greatly reduces the difficulty of constructing Lyapunov function by the traditional one. However, this method is sensitive to the order of system, and the dimension disaster will occur in higher-order systems.To solve the above problems, this paper explores an idea that, firstly based on the theory of multiple time scale the converter element model is divided into two subsystems of different time scales, and by solving the eigenvalues of the state equations, fast and slow subsystems are identified and then the former is ignored to realize the order reduction. The rationality of the order reduction method is verified by contrasting the dynamic characteristics of the original model and the reduced order model with time-domain simulation. Further, the Lyapunov function of reduced order model is constructed by Lyapunov function method based on Sum-Of-Squares optimization. Finally, the effectiveness of the method is tested on three-machine-ten-bus AC/DC system with the wind power module in matlab platform.