In aspheric surface detection based on the Zygo interferometer, the precision of the aspheric surface′s location along the optical axis (Z-axis) direction has direct effect on the accuracy of the measurement results. This article proposes a Z-axis positioning method for aspheric measurements based on the minimum PV value. The theoretical model of aspheric and reference spherical waves is evaluated, and the appropriate reference spherical wave radius, vertex deviation, and theoretical measurement location are developed. The positioning procedure along the Z-axis direction is completed within 0. 001 mm precision. The proposed method can precisely identify the theoretical measurement location and efficiently decrease the measurement error introduced by the positioning error. It can also precisely determine the reference spherical wave radius, which is essential for the point cloud reconstruction and stitching method based on the Zygo interferometer. The positioning of an aspherical mirror with a diameter of 108 mm is evaluated experimentally, and the positioning results are close to the calculated results of the point cloud data of the best reference spherical wave, demonstrating the method′s correctness. The PV residuals are 0. 047 λ and the RMS residuals are 0. 019 λ compared to the null testing results, which further demonstrates the positioning method′s accuracy.