A self-developed large aperture near-infrared phase-shifting flat interferometer was used to perform an absolute test for a large aperture SiC flat mirror in an oblique incidence condition. Firstly, the cavity wavefront data were obtained in a standard Fizeau configuration, and then the test flat was put into the cavity in its normal axis with an angle α to the optical axis of the interferometer to obtain the second wavefront data set. After processing the two data sets, the absolute vertical profile across the center of SiC mirror was also obtained.Finally, several absolute vertical profiles of the Φ630 mm SiC mirror were measured and the results indicate that PV and RMS of the center absolute vertical profiles are 0.061λ and 0.014λ, respectively. The experimental results show that this measurement device can achieve the absolute tests of vertical direction of flat mirrors with the effective apertures larger than that of the interferometer, and it is especially applicable for the absolute tests of optical sur-faces coated with high-reflection films or metal surfaces.