Abstract: Most of the underground hot strip reels, which are widely used in hot strip work, are importation or are made with introduced technique from overseas. The study on the design principle of hot strip reels is not deep-going. The selecting method for the diameter of withdrawal roll was discussed combining with the request of the national tackle key item. The calculation formulas of the diameters are provided in virtue of the elastic and plastic bending deformation theory. The calculated result turns out according with the practical evidence.