Abstract: An intelligent control system for the all-position and multi-layer welding robot of spherical tanks was developed. In this system a 2-point vision servo system was sued to guide the welding robot following a line parallel to the groove repeatedly. An industrial controller PLC was used as the CPU. In addition several auxiliar processing and driving circuits had been used to control the robot's 5 freedom parts such as the flexible magnetic wheel, the tracking mechanism and the swing mechanism to work coordinately. The welding robot under the control of this intelligent system can realize all-position and multi-layer welding of a ball tank or a cylinder tank. There are two patented innovation points in the all-position welding robot controlled by this system. The first is the flexible magnetic wheel used to make it possible for the robot to attach to and roll free on the tank surface without any rail. The second is the two-stage vision servo system, the mainfunction of which is to guide the welding robot and the torch to go along the welding line. The welding tests show that the welding robot can make the all-position and multi-layer welds with high tracking accuracy, excellent quality and reliable behavior.