To determine the surface profile of a sapphire substrate with the change of micrometer scale, a test based on oblique incidence is proposed to extend the measurement range of a interferometer. After studying the theory of oblique incidence measurement principle, the relationship between oblique incidence angle, surface error and wave-front aberration is derived. A simulation model is constructed by Zemax. The influence of surface error and incidence angle on the detection results is investigated. The key parameters such as the best incidence angle are determined in the simulation. The results are used to guide the measurement of 100 mm sapphire substrate on the Fizeau interferometer, the root mean square (RMS) and peak-to-valley (PV) results of the sapphire substrate are 5.182 μm and 1.251 μm. The influence of incidence angle on resolution, sensitivity factor and fringe contrast is discussed. The proposed method can extend the measurement range of interferometer. The result shows the method is suitable for the sapphire substrate with vector height less than 5 μm when the measurement angle is 70°. The error is less than 0.1 μm. The measurement range is upgraded by 1.46 times, and the suitable fringe contrast can be also obtained.