Abstract: In order to achieve the measurement of large-aperture mirrors with high precision, the stitching interferometer system is established, the whole surface is obtained. The basic theory of stitching measurement technology is to divide the whole test mirror into several parts, and then measure the surface of each part respectively, and finally stitch these surfaces of different parts together. In the process of stitching measurement, overlapping areas between different local surface shapes use mean method of fusing overlapping areas would reduce the high-frequency components. First of all, the original overlapping areas surface shape goes through the wavelet transformation, resulting in low frequency coefficients and high frequency coefficients; Then, low frequency coefficients and high frequency coefficients is determined by different fusion rules; in the end, Finally, overlapping area surface shape is obtained by wavelet inverse transformation. This paper discusses the stitching interferometer test on a 120 mm×40 mm rectangular reflection mirror, in which fusion of overlapping areas has average method as well as the proposed algorithm, and also uses power spectral density conduct an objective comparison on the fusion results. The result of this experiment shows that the proposed algorithm fusion has better effects to improves the retention of high-frequency than the conventional methods.