The invention relates to a ballscrew pair applied in a high-speed feed system with steep lead, high rotating speed, a rotating nut but a fixed leading screw, comprising the leading screw, the nut, a bearing steel ball and a ball-returning device, wherein, the nut is sleeved on the leading screw; the bearing steel ball is arranged in a steel ball spiral race consisting of a spiral race slot of theleading screw and a spiral race slot of the nut; mounting slotted holes with the same number as the number of turns of the nut race slot are uniformly distributed on the periphery of the nut; the mounting slotted holes are respectively provided with the ball-returning device; the inner wall of each ball-returning device is provided with quintic parabolic a steel ball circle bead groove; two ends of each circle bead groove are respectively communicated with two adjacent spiral race slots on the nut so as to form a closed steel ball endless chain. When the ballscrew pair works, each circle of the steel balls crosses the leading screw surface through guidance of the circle bead groove on the ball-returning device to form a circulation. The product has the advantages of long service life, high transmission efficiency and precision, good working stability and so on.