A rail type mobile robot introduced in this paper is used to carry out automatic welding on the T,Y and K-joints in offshore platform jackets.This robotic system is composed of a mobile platform and a serial manipulator.The structure of manipulator was designed according to the mathematical modeling of single brace tubular joint ; some improvements of the robot mechanism were considered to improve the flexibility of manipulator and increase the workspace. The inverse kinematics analysis was solved through two steps calculation. The collision avoidance strategy used in the K-joint automatic welding was discussed.Moreover,the simulation results were presented to demonstrate the feasibility of the robot mechanism used in tubular joint welding and the validity of the joint value calculation method.