Abstract: Annular sub-aperture stitching interferometry technology can test steep conformal aspheric surfaces with low cost and high efficiency without auxiliary null optics. The effective splicing algorithm was established based on simultaneous least-squares method and Zernike polynomial fitting. Firstly, the basic principle of the algorithm of the annular sub-aperture stitching was studied. Secondly, the mathematical formulas of the sub-aperture effective area were derived and the parameters of the sub-aperture effective area were calculated and optimized. Finally, the reasonable mathematical model was established. The detection method of annular sub-aperture stitching high steepness aspheric has been tested through on experimental verification. As a result, the surface map of the full aperture after stitching was consistent to the input surface map, the difference of PV error and RMS error between them is 0.015 1 and 0.004 7( is 632.8 nm); the PV and RMS of residual error of full aperture phase distribution is 0.043 5 and 0.005 2. The results conclude that this splicing model and algorithm were accurate and feasible.