The structure features and work principle of threeroll centering device of the Φ340 mm unit punch in a company were described.The device powered by two cylinders was capable of three major movements, i.e., ejector rob,capillary bits and rollopening, so as to ensure the axial stability requirement of the ejector rob in the process of the steel tube perforation process. And the defects exposed in the applied process were introduced and the defects of the part parts were improved.The actual operation result shows that the system is able to fully meet the piercing process requirements of seamless steel tubes, which is worthy to be used on similar mills.