A real-time interferometric method for testing asphere was proposed. It can measure aspherical surface of small asphericity without other null optics. When the tested asphere is close to a sphere，a sphere mirror is used as the reference surface. The phase distribution can be obtained by the the digital wavefront interferometer. The surface error was obtained by substracting the theory wavefront error from the phase datum and eliminating the translation error though least-squares fitting. Meanwhile the experiment was carried on with a 350mm aspheric mirror by this method. As results

the PV and RMS of the surface error was 0.387λ and 0.048λ

respectively; the surface map and interferogram were consistent to the surface map and interferogram tested by null compensation; and the difference of PV and RMS error between them was 0.033λ and 0.006λ

respectively. The results conclude that this method is feasible and accurate.