The utility model relates to a positioning device for a grinding machine, which comprises a positioning air cylinder and rolling wheels. The positioning air cylinder is used for pushing a glass substrate 1 to move; the rolling wheels are arranged on the lower part of the glass substrate 1; a hollow plate is arranged blow the rolling wheels; the upper part of the hollow plate is provided with air holes; and the hollow plate is communicated with the output port of an air pump. According to the technical scheme of the utility model, because clean air is blown blow the glass substrate 1 to separate the glass substrate 1 from the rolling wheel below the glass substrate 1, the scuffing caused during positioning is avoided, the quality of the glass substrate 1 is guaranteed, and the yield is improved.