The principle of stitching interferometry for testing the flat wavefront was introduced. Error averaging method was used in the experimental data processing, which could reduce error accumulation and improve the precision. Based on the algorithm, we carried out experiments, tested the telescope optical system simulated by Zemax software, and used 100 mm small aperture interferometer to measure the 200 mm flat mirror. Compared the stitching results with the directly measured full aperture, the Peak-to-valley (PV) and Root Mean Square (RMS) of the wavefront phase distribution residua are 0.015 1λ and 0.001 6λ, 0.055 9λ and 0.000 4λ respectively. It was proved that the model and method are not only accurate and feasible to test the optical mirrors, but also can be applied to test optical system.