Through analyzing a simple rotary cutting device for regular polygon，the forming principle was introduced, and the equation of knifepoint relative to workpiece was established. Then, the reason for length error of the polygon and the method of modifying were discussed along with straightness error. Hence, the precision of regular polygon cutting on rotary cutting machine tool gets assurance, and the work efficiency increases. By programming with VB, the dynamic simulation for locus of knifepoint was performed. Furthermore, the modeled curve of knifepoint，which proves the locus of regular polygon realized on rotary cutting machine tool can be actualized. 