The positions of dot light source of incident spherical wave and reference spherical wave are directly related to the selection of CCD, optics, analysis of interferogram and the evaluate accuracy of a testing system for aspheric surface. In order to predefine the best position of sources of incident spherical wave and reference spherical wave, a new method is adopted to fix dot light source of spherical wave by calculating and analyzing the fringe density. By using the above method, the best source position of spherical wave not only can be predefined but also the interferogram be fully analyzed, simultaneously the specific strategies of selecting CCD and optics can be determined. The results of the research also can be used as a theoretical guidance debugging in the process of testing aspheric surface.