Abstract: The paper introduces operational principle, interpolation algorithm and software implementation design of saddleshaped auto-welding system.By x, y, z three-axis interconnected, the system controls welding torch completing auto-welding of saddle welding seam.Based on two dimensional circular interpolation of angle approaching, the geometrical characteristic of saddle-shoped curve was analyzed, and a interpolation algorithm by geometrical relation of x, y, z three coordinate was designed.Algorithm makes use of the whirliglg for the projected footprint in the xoz plane of saddle-shaped curve to fit saddle-shaped curve, which desolves the problem of auto-welding for saddle welding seam consequently.The algorithm is so samiple that it can be easily realized by programme with minor error and high efficiency.