For the purpose of validating a new stitching algorithm—Sub-aperture Stitching and Localization (SASL) algorithm,a measuring and figuring experiment was described for the SiC flat mirror with elliptical shape of 225 mm in major axis and 161 mm in minor axis.A comparative experiment of sub-aperture stitching test with full-aperture interferometry on a 100 mm diameter flat mirror was presented in order to determine the algorithm parameters and the precision requirement of stitching equipment.Following this,a sub-aperture stitching process for the SiC flat mirror was carried out,which contained 5 iterations of Ion Beam Figuring (IBF) process and a full-aperture testing.In this process,sub-aperture stitching test offered the surface error data precisely to IBF,which ensured the quick convergence for the process to achieve a final surface RMS error of 50 nm.The experimental results show that SASL algorithm relaxes the precision requirement for the stitching equipment and reduces the influence of stitching process on the results.