Abstract: A continuous trajectory mathematical model was established for offshore platform jacket welding robot. The robot with 5 degrees of freedom has enough workspace to finish automatic welding of Y joint and flexibility to meet the requirement for obstacle avoidance in K-joint welding. The kinematics of the robot was built according to the robot structure. Since this robot has a redundant degree of freedom,the joint value was calculated by two steps methods. To ensure the motion stability of the robot,the joint velocity was calculated by derivation operation of joint value through limited method. The simulation result shows that this robot can achieve automatic welding of the K,Y, T joint,and the mathematical model of joint value and velocity can be used to control this robot.