In order to describe the difference between aspheric surface and sphere accurately and fit the aspheric surface measurement techniques, a best-fit spheric sureace based on slope asphericity is proposed. First the min-max slope asphericity associated to manufacturing difficulty of computer-generated hologram and aspheric surface measurement difficulty was analyzed, when the min-max slope asphericity is beneficial to reduce the measurement error of laser deflectometry. Then a calculation model of best-fit sphere is proposed, and the distribution of slope asphericity corresponding to different comparing spheric surfaces, the best-fit spheric sureace of different aspheric surfaces, the effects of aspheric surface parameters to calculation result, and the best-fit spheric sureace of different asphericity definitions are analyzed. The results indicate that the center of best-fit spheres corresponding to different aspheric surfaces is the cross point between the normal of 0.85～0.87 aperture of aspheric surface and the optical axis, thereby the calculation process is optimized, and the best-fit sphere based on slope asphericity is different from prior methods, thus it fits aspheric surface measurement.