By calculating the interference fringe density of aspheric surface reflecting optical wave and a spherical optical wave, a new method for determining asphericity and the best-fit spherical surface of aspheric surface has been discovered. This method applies computer digital analysis technology to directly compute interference fringe density that the fringes are produced by interfering with aspheric wave and spherical wave in different radius. The spherical surface that is of the least fringe density in the most densities above is best-fit spherical surface of aspheric. The prime advantage of this method is that the radius of the best-fit spherical surface, the asphericity, the density and positions of interference fringe can be well determined not to require any analytical calculation for aspheric surface function.