Abstract: The idea that adopts decreasing distribution method to design the sizing mill passes is proposed in order to use one common set of passes to roll the 30~100 mm heavy wall,large-sized steel tubes with the large-sized 5-stand sizing mill.Elaborated here in the article are the main aspects of the pass design,involving the design approach with the reduction rate,the computing method for the steel pipe thermal expansion coefficient,the general reduction rate decreasing distribution mode,and broad siding determination as well as the pass design.The scheme of the sizing/reducing passes as designed with the decreasing distribution method is reasonable,being satisfactory to the requirements for manufacturing the 30~100 mm light-wall and heavy-wall pipes with one common set of passes.