Abstract: Described here in the article are the pass design principle and method for the mandrel pipe mill.Also elaborated are the design method for the elongation coefficient distribution of the pass top,and the design thinking of the piercing mill pass.It is concluded via the analysis that the above mentioned design method features simple calculation,and the pass as designed with the method is capable of satisfactorily serving the steel tube production;and the 135 mm pass series as designed with the method ensures stable manufacturing process,and keeps the wall thickness tolerance of the steel pipe as rolled within ±5%.