Abstract: The parameters such as wall thickness upset rate, diameter reduction rate, wall thickness coefficient, diameter reduction temperature, stretch force are measured in practice on 12-stand 3-roll mini-stretch reducing mill and the effects of these parameters on the wall thickness accuracy of typical varieties of seamless steel pipes are analyzed and studied. The math model formula for the calculation of non-uniform wall thickness is derived from the computer-aid regressive processing and the math model proves to be effective on the spot in practice.