The stability of a power system has always been of concern. Now, wide-area measurement systems are used widely. In this paper, we focus on supplementary turbine/governor control to enhance the stability of a power system with wide-area measurement information. An H∞ supplementary turbine/governor controller based on the multi-agent method is introduced to enhance power system stability. First, the synchronous generator is modeled as a second-order agent. The information of communication topology and power network topology are used to build the system model and the controller design. Then, a linear output feedback controller is adopted. With a suitable Lyapunov function and by solving the linear matrix inequality, the parameters of the controller are tuned by the H∞ synthesis method. Finally, numerical simulations are presented on a Western Systems Coordinating Council three-generator nine-bus power system, validating the proposed design.