Abstract: There are some errors when big frame intersection line welded structural assemblies are produced,so the trace of seam is both instructed and fitted as the robot is used,in which quality of welding is decided by the precision of fitting.Focused on saddle-back welding seam,ellipse fitting and three-coordinate axis separated least square algorithm fitting are both designed.Simulation based on some teaching points in key position is made,and the result shows that the three-coordinate axis separated least square algorithm fitting is more precise than the ellipse fitting.It is more fitted for big intersection line welding frame.