By calculating the interference fringe density of the tested aspheric surface reflected wave and reference spherical wave in the CCD to record interference pattern, the distribution of fringe density is gotten when source of reference spherical wave is in different positions. To compare the maximum fringe density, the position of source of reference spherical wave while the maximum fringe density is the smallest in CCD is the position of best-fit spherical surface. The spherical wave emitted in above position is the best reference spherical wave. At the same time, the maximum fringe density is also the important basis for choosing CCD in system of testing aspheric surface by digital holography, which is an important technology parameter to evaluating accuracy of a testing system for aspheric surface. The method can be used to fix on best position of reference spherical wave source for inspecting any aspheric surface, which does not require any analytical calculations about aspheric surface function.