Abstract: The space structure of the rotary expanding pipe mill is different from conventional rotary pipe mills.To get a better understanding of the regularity of the mother tube's dimensional variation along the deformation zone during the rolling process of the rotary expander, it is required to calculate the distance between the roll surface and the rolling centerline, particularly the minimum distance, i.e., the pass opening.Based on the special space coordinate relationship of the rotary expander, the pass opening between the mill roll surface and the rolling centerline is determined by means of the planar section method.As a result, the computation model is established for dimensional design of the mating plugs.