In this paper, an expert PID controller used for digitally controlled DC-DC buck converters is proposed. It is designed bycombing expert theory with traditional PID theory, basing on the controlled objects and the knowledge of control laws. The controller uses the expert experiences to regulate the PID parameters online according to the dynamic characteristics of the system. The simulation and practical results indicated that, comparing with conventional PID controller, the expert PID controller which has faster startup transient response, smaller overshoot and more stable steady-state response, is better than conventional PID controller. It is well fit for the demand of control system.