According to the low degree of welding automation on complex curve, the paper designs the numerical control system of automatic welding. The system uses PCL6045B motion control chip, the most control 4 axes, and the system can weld the complex curve line, improve the welding efficiency and welding precision. The system has built-in soft PLC, and the soft PLC can perform automatic control on welding machine, fixture and other peripheral mechanism. Feedback from its application on welding machine indicates that the system works reliably.