When machining listing points curves using CNC technology,firstly it is required to acquire cubic spline function equation of the listing points，and then approach the cubic spline curve with short line or short arc. An improved approaching method using the determining step size was presented through researching the methods of approaching of cubic spline curve.It can approach the cubic spline curve by straight lines.This method is simple and can generates less NC codes. Accuracy of this approaching method can be controlled. Numerical control system was developed based on this method in the VC++ environment .The effectiveness of this method was verified by an example.