Aiming at the problem of power angle stability in power systems, an energy storage sliding mode controller was proposed to control the acceleration power of synchronous generator based on the detailed modeling and analysis of the interaction between energy storage and synchronous generator energy. Using the strong robustness of sliding mode control, the speed deviation of the synchronous generator was introduced into the sliding mode surface. The acceleration power of the synchronous generator was controlled to decouple the synchronous generator from the power grid in control. The frequency of the synchronous generator was restored to the rated value. Thus the power angle stability of the system could be enhanced. A nonlinear smooth function was used to replace the sign function in the sliding mode control, then a practical sliding mode controller was obtained to weaken the chattering phenomenon in the sliding mode control. The stability of the proposed controller was analyzed by constructing the Lyapunov function. In the Matlab/Simulink, the performance of the system under disturbance was compared among the parameter feedback linearization controller (PFL), the power oscillation damping controller (POD), the flexible controller (FC) and the proposed method. The results show that the proposed controller has a stability time shortened by more than 30% compared to the aforementioned three methods.