Abstract: A method for fast testing aspheric surface with small asphericity (less than 0.01 mm) was proposed, which does not need compensation system or other optical elements. First the practical wavefront aberration was measured by phase-shifting interferometer, and the adjustment errors were eliminated. Then the theoretical wavefront aberration of aspheric surface on-axis or off-axis was calculated based on the equation of aspheric surface. At last the residual wavefront aberration (surface error) was obtained by subtracting the theoretical wavefront from practical wavefront. A hyperboloidal mirror with the diameter of 135 mm was measured by this method, meanwhile this mirror was tested by null compensation; the tested results by these two methods are similar. It concludes that the method of digital wavefront for testing aspheric surface is feasible.