To ensure the stability of units and systems under the new power system, a design method of interconnection and damping assignment passivity-based controller for steam turbine speed control system is proposed. Firstly, based on the theory of Hamiltonian system, the dynamic process of power system is described from the perspective of energy, and the influence of the structure of Hamiltonian system on the system energy is analyzed. Then, the variable gradient method is used to construct the system energy function including the speed control system. The damping matrix of the system is determined with the constraint of minimizing the derivative of injected energy. The difficulty of determining the system damping matrix in the design process of the interconnection and damping assignment passivity-based controller is overcome, and the design of the interconnection and damping assignment passivity-based controller is completed. Finally, the proposed method is used to build a model of single-machine infinity bus system in MATLAB/Simulink, and the effectiveness of the controller designed in this paper is verified.