Abstract: Presented in the article is a CAD illustrative method for groove opening of rotary piercing mill,which is much simpler and easy-to-use than the mathematic method as used for the same purpose.After computer simulation of the piercer roll space position,a perpendicular plane is made from a certain point in the rolling line,and then following the assumed conditions for the opening value,the opening value is approached to by means of drawing on the said plane.With this rather simple illustrative method,certain accuracy of the groove opening value can be obtained to meet actual engineering requirement.In addition,a brief analysis of the said groove opening data is made.