Abstract: The pass gap of the piercing mill is formed by the plug rolling section and the roll outlet cone section,and the uniformity of this gap directly affects the wall thickness accuracy of the shell.An optimization model for the wall thickness accuracy of the pierce shell is developed based on the computer numerical computation.With the model not only the piercer pass gap can be computed out precisely,but also the design of the plug rolling section can be optimized.Moreover thanks to the model,based on consideration of relevant elements like the plastic deformation of the plug bar as developed during the actual operation,and the adjustment of the plug lead,etc.,the toe angle compensation method can be adopted to guarantee the shell wall thickness accuracy,which shows the strong practicability of the model.And applying the model to the actual production can keep the cross sectional wall thickness accuracy of the shell with ±4%.