Abstract: The paper analyzed working principle of fly-back converter and compared working state in current continuous mode with the one in discontinuous current mode of fly-back converter. It introduced a design method of RCD clamp circuit of switch power in details and selected devices and parameters through calculation and simulation. The results of Matlab simulation and experiment analysis showed that DS voltage wave of switch tube is improved significantly and voltage spikes are reduced more than 60% when RCD clamp circuit is joined and suitable R,C value are selected, so as to effectively reduce damage and stress requirement of switch tube and decrease power loss of switch tube.