Abstract: Described here in the paper are the requirements for geometric dimensional accuracy of the heavywall drill-purpose pipe.Also analyzed on the basis of the process characteristics of the Assel pipe mill are the control difficulties and critical points concerning manufacturing the pipe.The analysis result reveals that in case of producing the said pipe with the Assel mill，it is rather difficult to set up the reduction amount distribution and the rolling parameters of the slight-stretch sizing（reducing） mill stand，and high inner diameter tolerance is required due to big reduction amount that causes dramatic trembling during the rolling operation.It is possible to improve on both the W.T.and O.D.accuracies of the heavy-wall drill-purpose pipe so as to meet the I.D.requirement by means of employing reduction-rolling process to distribute the reduction amount to the three main rolling units， i.e.， the piercer，the Assel mill proper and the sizing mill，decreasing the running speeds of the piercing mill and the rolling mill proper，and adding preventive guide bush device，etc.