Abstract: The principle, system structure, interpolation algorithm and its simulation of an automatic welding system for saddle-shaped curve were introduced.An automatic welding system was designed with four axes: τ, y, ρ and σ.Through the coordinated feeding of the four axes, the trajectory and pose of the welding torch can be controlled to implement the automatic welding of saddle-shaped curve weld.Based on the cinematic analysis and the principle of the space analytic geometry, the trajectory model and the welding torch pose model were established, and a four-axis interpolation algorithm for automated welding of saddle-shaped curve weld was proposed.The simulation of the algorithm shows that the algorithm has ideal trajectory and welding torch pose control precision.A direct trajectory control can be realized with this interpolation algorithm.