Freeform optical surface is a new generation of ideal optical element because it can effectively correct image aberrations and greatly simplify optical system structures. Because of its high gradients and strong deviations from ideal sphere, it is extremely difficult to measure freeform optical surface with sufficient accuracy, which is the key limitation on its fabrication and application. Therefore, a novel non-null interferential method based on the use of multiple test beams for illuminating freeform surface with different angels is presented, which can compensate local gradients of freeform optical surface. A reverse optical design method based on “black box” theory is proposed, which can resolve the problems caused by non-rotational asymmetry surface. A real non-null interferometer for testing freeform surface is designed, and an eyeglass with non-rotational asymmetry surface is measured successfully, which verifies the feasibility of the system.