Saddle shaped curve in the industrial production is widely used in the welding, and the traditional interpolation algorithms cannot be applied to the automatic welding. In this paper, based on the Mitsubishi PLC controller Q02HCPU and the interpolation algorithm principle of design research, the saddle-shaped geometry characteristic foundation, proposed based on point by point comparison and angle approximation of direct interpolation algorithm, and through MATLAB software simulation. The experimental results show that the algorithm has high precision in interpolation, and make software programming easily to realize, to improve the saddle shaped welding production automation.