Abstract: The full-digitized control system of inverter-controlled arc welding power supply with pulsed variable polarity output based on dual DSPs (digital signal processor) is developed by using DSP56F805. To overcome the influence of the pulse width holding and delaying existed in digital PWM (pulse width modulation) control pulse on the output performance, such as the dynamic response and static precision of power supply output, the current pre-estimating algorithm is proposed based on the LR (inductance and resistance) model of power supply output, and then the straightforward digital PID (proportional integral differential) control algorithm is modified. Meanwhile, the anti-disturbance characteristics of digital control system is improved effectively by pre-treating on the detecting signals of current and voltage, A/D sampling in phase with PWM control pulse, digital filtering, etc. The correctness and effectiveness of digital PID control algorithm based on current pre-estimation is validated by the control experimentations on the pulse variable polarity output of power supply.