The limitations of current interferogram reduction software with circular Zernike polynomials in analyzing obstructed interferogram are illuminated. The Zernike circle polynomials are no longer orthogonal over an annular region and have no explicit physical meanings. The Zernike annular polynomials are discussed and illustrated,which have orthogonality and the similar natures with the Zernike circle polynomials. The experiment is carried out with a Φ 300 mm aspherical mirror with 0.23 obscuration by null-compensation,and the obstructed wavefronts are fitted with Zernike annular polynomials. The first four terms are regarded as misalignment errors and eliminated from the original phase distribution directly,and thus an accurate surface map is gained,which provides a guarantee for the superprecision fabrication.