A novel inverter power source is developed characterized with constant output current and unit power factor input.Digital signal processor (DSP) is used to realize power factor correction and control of back-stage inverter bridge of the arc welding inverter. The fore-stage adopts double closed loop proportion and integration (PI) rectifier technique and the backstage adopts digital pulse width modulation (PWM) technique. Simulated waves can be obtained in Matlab/Simulink and validated by experiments. Experiments of the prototype showed that the total harmonic distortion (THD) can be controlled within 10% and the power factor is approximate to 1.