Abstract: Analyzed are the structural principle and function characteristics of the 4-roll centering device of the piercing mill.The design schemes of the position close-loop control hydraulic system and the series synchronous control hydraulic system are compared to each other.Based on the comparison result，an economic，reliable and high efficient hydraulic system for the said 4-roll centering system is designed and developed.Actual operation data of the system demonstrate that the said hydraulic system is capable of long term，non-failure，maintenance-free，self-correcting and high-efficient operation，having completely met the functional and technological design requirements，and brought about satisfactory economic profit.