Abstract: Based on the conjugate geometric relationship between work-rolls and workpiece in the rolling zone,a new method for designing plug of the two-high rotary piercer and respective deduction-calculation formula were derived, Also described in the paper are the principle for designing the working section & reversal taper section of the plug and some examples thereof.It has been proven by trial commercial production that With the piercer plug designed by means of the said new method,much higher radial wall thickness accuracy is available.