Abstract: The U-I mode inverter P-MAG welding machine controlled by 80C196KC 16-bit single chip computer was designed.By sampling the voltage and current of arc in the welding process,the arc voltage is kept constant in the peak period and welding current is kept constant in the base period.Arc length self-adjustment was realized by changing of the peak current,and the pulse parameter was optimized to make sure the one pulse one droplet transfer mode.The experimental result indicated that the welding machine has perfect welding performance.