Abstract: Described are the structure features and work principle of the hydraulically driven 3-roll centering system(steadier)of the piercing mill.The system,powered by a specially designed dual-stroke hydraulic cylinder,is capable of the three major movements,i.e.,roll-closing to the bar,roll-closing to the shell and roll-opening,so as to ensure the proper axial delivery of the shell during the pipe piercing process.The actual operation result shows the said system is able to fully meet the piercing process requirements of seamless steel tubes,being worthy to be used on similar mills.