The copper - aluminum pipe has been widely used in the refrigeration industry. And the welding quality becomes the focus. A PLC control system was developed to accurately control the resistance welding process of the Cu-Al pipe. The welding voltage was adjusted by a couple of silicon controlled rectifiers (SCR). To restrain the effect of network voltage fluctuation on the welding heat generation, it was obtained that the experimental relationship of the angle of flow and the network voltage, and fitted to a quadratic polynomial equation. Therefore, the control system can calculate the exactly the angle of flow value according to the current network voltage. The experiment results show that the error of the welding heat generation is limited in 5% when the network voltage fluctuates in the range of 380V ±10%. And the SEM and EDS analyzing results suggests that the welding seam length is more than 5mm, and there is hardly eutectic composition.