Trajectory planning of mobile welding robot is the basis for trajectory control.It important part of mobile welding robot system . In order to improve the trajectory planning efficiency and accuracy of multi- joint mobile welding robot,the paper propose a method of combination of gradient descend and dichotomy consider of feature of multi- joint mobile welding robot. multi- joint mobile welding robot composed of a manipulator and a mobile platform has a much larger work space than a fixed-base manipulator due to the mobility provided by the platform. In recent years, the study of mobile manipulator systems have aroused rapid development in academia and industry. firstly,kinematic model was built, and propose the method of combination of gradient descend and dichotomy design process. After that, sine curve was chosen as typical input, and demonstrate technological application values and high feasibility