Abstract: According to the basic characteristics and the needs of arc welding/cutting power source, such as load current, short current and unload voltage, considering to the mains voltage fluctuation, economical and person security, the inverter arc welding/cutting power supply with composite control of sliding mode control(SMC) and PI is researched and designed.Through demonstrating the external characteristic demands of welding/cutting power supply and analyzing the control algorithm, PI control is used on current loop and SMC is introduced on voltage loop, which has not only effectively solved the voltage overshoot, but also realized a faster voltage resume to pilot arc again quickly.The control algorithm of phase-shift full-bridge indirect SMC based on the average state space model is deuced theoretically, and a direct phase-shift PWM wave generation method is applied, which makes the control more practical and simpler.Some experiments on a 20 kW arc welding/cutting power source are experienced between the composite control of SMC and PI and the single PI control.The results prove the effectiveness and robustness of the composite control SMC and PI.