The invention relates to the field of numerically controlled polycrystalline double-end-face grinding machines, and particularly relates to a centering device for grinding machines. A centering device for numerically controlled polycrystalline double-end-face grinding machines comprises a bracket, a fixed seat, a sliding seat, a lifting cylinder, a horizontal pneumatic claw and a jaw assembly, wherein the fixed seat is fixedly arranged at the top of the bracket, the sliding seat is arranged on the fixed seat, and the sliding seat and the fixed seat are matched so as to form a moving pair; the lifting cylinder is fixedly arranged on the fixed seat, and a cylinder rod of the lifting cylinder is fixedly connected with the sliding seat; and the horizontal pneumatic claw is fixedly arranged at the bottom of the sliding seat, and the jaw assembly is arranged on the horizontal pneumatic claw. According to the centering device for numerically controlled polycrystalline double-end-face grinding machines, disclosed by the invention, through the cooperated positioning of the lifting cylinder and the horizontal pneumatic claw, after the centering device is applied to an operation process of a numerically controlled polycrystalline double-end-face grinding machine, the positioning of a processed workpiece is more accurate, thereby reducing the operation difficulty; and the automated control of positioning is realized, the processing accuracy is improved, and the production cost is reduced.