In this paper, a controller based on eqivalent circuit model is proposed for a wireless power transmission (WPT) system with BUCK converter. The mutual inductance of transmitter and receiver coils can be obtained effectively by the model. The controller can regulate the voltage output on load resistance while mutual inductance is changing periodically in the dynamic WPT system. The simulation results by PLECS show that the proposed novel control rule for the BUCK converter serised by WPT resonant circuit has better performance and robustness by comparing with a PID controller.