In this paper, the principle of subaperture stitching interferometry was introduced. A testing stage with five degrees of freedom for stitching interferometry was built. A model based on least-squares method and error averaging method for data processing was established, which could reduce error accumulation and improve the precision. A 100 mm plane mirror was measured with a 50 mm aperture interferometer by means of stitching interferometry. Compared with the results by a 100 mm interferometer, peak to valley (PV) and root mean square (RMS) of the phase distribution residual are 0:0038l and 0:0004l, respectively. It proved that the model and method are helpful for large optical measurement.