Abstract: According to the welding process of copper pipe and aluminum pipe, a PLC control system with the function of voltage compensation was developed to stabilizing the welding electric current and welding quality.Before welding, power voltage was measured, and the angle of flow for silicon control can be calculated by control system, and then transmitted to the trigger circuit board. And the cubic spline was used to fit the relationship of the angle of flow and power voltage.When the voltage waved in the rang of 380 V±15%,Φ8×1 copper pipe and aluminum pipe were jointed and the experimental datum of welding electric current was obtained.The result of the T-test to electric current datum shows that the welding current is stable.The welding quality is reliable.