This paper develops a portable all-position welding robot for the welding of intersected pipes. The mathematical model of welding seam has been established and a complete procedure is adopted to conduct the design. The task and motion of the robot are analyzed and a mathematical description of the pose and position of the welding torch is given. Based on that, a new kind of 5 degree of freedoms welding robot has been designed with mixed method of series and parallel for satisfying welding seam of intersecting pipes. An automatic centering robot trick with 2 degree of freedoms has also been designed, which realized the control of torch position and posture separately. The welding result indicates that the anchor is surely secure and accurate and the control method can satisfy the technological requirements of welding seam of intersection.