Abstract: Firstly, the configuration of welding mobile robot is introduced, then algorithm and implementation of auto searching weld line is presented on the basis of kinematics model of the robot, at last trajectory planning among auto searching weld line is investigated in detail. The experiment result shows that the developed welding mobile robot can successfully implement the task of auto searching weld line before welding, and the tracking error can be controlled within approximate ±1.5mm, satisfying the requirement of practical welding project.