In this Letter, a test method based on oblique incidence is practically implemented in the interferometric measurement process. Three sets of wavefront data are achieved through cavity interference measurement with a Fizeau interferometer and one oblique incidence measurement. An iterative algorithm is applied to retrieve the absolute surface shape of the test flat. By adding two sets of measurements, the absolute surface error of the interferometer’s reference flat can be obtained. The new method can not only calibrate the reference flat error of interferometer, but also provide the absolute measurement method for high precision optical components applied in high power laser systems.