Automatic welding equipment with arc length control for electrolytic aluminum plant is  designed. The servo system is composed of AC servo motor, ball screw, electro-gas (EGW) power, base material to be welded and servo controller. The welding power is installed of numerical communication port of welding current, providing information of velocity error between welding pool and gun. MIMO model of welding process is simplified into a linear error amplified. Dynamics of AC servo motor and ball screw with external load are analyzed. And a list of parameters is provided for further controlled design. The system is essentially a multi-loop control system. The internal loop of PID controller is tuned first. And then the whole system with PD controlled is tuned, so that the overshoot ( %) and stabilization time (ts) is comfortable. At last, with the help of Simulink, arc control performance test is conducted. It proves that the system can keep arc length almost constant. This method is practical and easy to be realized for industrial application.