Abstract: Based on the established rotary-rolling/piercing theory system,two piercing/rolling theoretic principles are put forward here in the article,i.e.,No.1:under the condition of proper biting-in,the larger the roll gap is,the better it is;No.2:properly controlling the piercing cavity effect.Analyzed are the effectiveness of increasing the billet diameter and reducing the outlet cone angle of the piercer,when using the large-sized expanding piercing process to roll the alloy steel tube with the small-sized seamless steel tube mill.Moreover the design method for the piercer inlet cone is described.Also elaborated is the design idea about applying rather small outlet cone angle or the one-section inlet cone piercing roll to the large-sized expanding mill.It is emphasized that keeping promoting and further completing the piercing theory system construction is one of the effective approaches to improving the rolling quality of the seamless steel tube.