Abstract: A mathematic model of the plug toe angle of the cone type piercer is established in order to precisely get the plug toe angle determined.And the design verification is conducted with the finite element(FE)method.As a result, it is identified that using the plug toe angle of the piercer as designed with the lean design method will be able to produce shells with wall thickness relative error as 2%~3%, and maximum deviation within 1%, being up to applicable API specification, and having rather high accuracy.The said design method provides relevant mathematic theoretic basis for establishing the tool design software, and the process control model for the piercing mill, so as to further sharpen the product competitive edge of the company.