Abstract: This research is mainly addressed on solving the weld seam tracing problems existing in design of wheel-mobile-robot based spherical tank welding robot. Generally, the moving route of wheel-mobile-robot can not be controlled accurately in real-time, however, weld torch must precisely moves along the weld seam at given speed to guarantee the weld quality of spherical tank. To solve this conflict, the moving mechanical structure of wheel-mobile-robot and weld torch,which is under certain constraint condition, has been designed. A weld seam route detecting system based on two CCD sensors has been established. After the dynamic analysis about the motion of wheel-mobile-robot on spherical tank, a control model for weld seam tracing of wheel-mobile-robot and its control strategy have been developed. Then, according to the constraint relations between the motions of wheel-mobile-robot and weld torch, a control algorithm which could control the weld torch position accurately in real time has been put forward. The exploited spherical tank welding robot has high weld torch tracing accuracy(up to ±0.5 mm), and has been successfully applied in the automatic manufacturing of spherical tank.