All-position robots are widely applied in the welding of complicated parts. Welding of intersecting pipes is one of the most typical tasks. The welding seam is a complicated saddle-like space curve, which puts a great challenge to the pose planning of end-effector. The special robots designed specifically for this kind of tasks are rare in China and lack sufficient theoretical research. Inthis paper, a systematic research on the pose planning for the end-effectors of robot in the welding of intersecting pipes is conducted.First, the intersecting curve of pipes is mathematically analyzed. The mathematical model of the most general intersecting curve of pipes is derived, and several special forms of this model in degraded situations are also discussed. A new pose planning approach of bisecting angle in main normal plane (BAMNP) for the welding-gun is proposed by using differential geometry and the comparison with the traditional bisecting angle in axial rotation plane (BAARP) method is also analytically conducted. The optimal pose of the welding-gun is to make the orientation posed at the center of the small space formed by the two cylinders and the intersecting curve to help the welding-pool run smoothly. The BAMNP method can make sure the pose vertical to the curve and center between the two cylinders at the same time, therefore its performance in welding-technique is superior to the BAARP method. By using the traditional BAARP method, the robot structure can become simpler and easier to be controlled, because one degree of freedom (DOF) of the robot can be reduced. For the special case of perpendicular intersecting, an index is constructed to evaluate the quality of welding technique in the process of welding. The effect of different combination of pipe size on this index is also discussed. On the basis of practical consideration, selection principle for BAARP and BAMNP is described. The simulations of those two methods for a serial joint-type robot are made in MATLAB, and the simulation results are consistent to the analysis. The mathematical model and the proposed new pose-planning method will lay a solid foundation for future researches on the control and design of ali-position welding robots.