Aiming at the problem of undersampling in the aspheric surface point diffraction interference measurement, a method for solving the location of the center of the best fitting spheric surface is proposed, to reduce the interference fringes density. The method is based on the minimum reflex convergence spot radius. Firstly the important role that obtaining the location of the center of the best fitting spheric surface plays in the high-precision aspheric surface measurement is analyzed. And taking minimum reflex convergence spot as the criterion to judge the location of the center of the best fitting spheric surface is pointed out. Then the computing model for the location of the center of the best fitting spheric surface is derived with ray tracing and iterative. Also the locations of the center of the best fitting spheric surface based on different asphericity definitions are compared. And the effects of aspheric surface parameters to the locations of the center of the best fitting spheric surface and the convergence spot radius are analyzed. The results indicate that, the best fitting spheric surface parameters based on minimum convergence spot radius is suitable for the aspheric surface point diffraction interference measurement model, also can be used to guide the assembly and debugging of the interferometer and the selection of wavefront reconstruction parameters.