In NC processing, only the straight line and circle arc can be processed directly , and the others must be fitted with them before processed. There are many methods of fitting curves with arc,but different shortage exists. An arc is built based on its length equal to the segment length of the curve approached ,the direction of the circinal center is judged according to the variety inclined of curve segment slope，so that convex curve is replaced with a convex arc.The overstep equation of triangle is simplified with expanding series to improve computing efficiency. The computation result shows the method is not only very simple to calculate,but also has high accuracy to approach the processed curve by an equal length arc.
