Abstract: Described here in the article are the princi ple and calculation method for designing the conventional pass;and also analyzed are the mechanism of and the control measures for the“inner hexagonal”bore of the medium-heavy wall steel tube as developed during the stretch-reducing process.Based on the production status of the Ф89 mm 3-roll mandrel mill production line,a design solution of the double circular arc pass is proposed.The finite element(FE)method is used to simulate the rolling process with the first section circular arc pass and the second one to produce steel pipes with the same size.It is regarded via relevant analysis that thanks to the said double circular arc design solution,the difficult as met in processing the pentagonal pass of the internally-driven stretch-reducing mill is solved.And both the FE analysis and relevant actual use result lead to such a conclusion that the above mentioned double circular arc pass is advantageous in restraining occurrence of the said defective“inner hexagonal”bore,and controlling the outer surface quality,being an innovative pass design of the 3-roll stretch-reducing mill.