Abstract: Welding of cylinder pipes is one of the common pipe connection, but the line of intersection is a complicated space curve. Based on a special welding machine, it introduces a real-time interpolation algorithm with controllable step length for line of intersection in automatic welding. The concrete method is that using liner portions with same length to approximate the complicated space curve under the conditions of keeping constant welding speed and satisfying the given approximation error. It takes the general connection form of cylinder pipe for example to illustrate the algorithm, and then OpenGL is used to simulate the composite motion track. Simulation results show that the real-time interpolation algorithm is feasible.