In this paper, the fitting method of cylindrical mirror with rectangular aperture is studied. The extended Zernike polynomial and Chebyshev polynomial in rectangular aperture are used as the polynomial of surface fitting respectively. The wavefront is reconstructed from the surface data. The corresponding relationship between Seidel aberration and the fitting polynomial is analyzed and compared. The Chebyshev polynomial shows the good ability of separating aberration than the extended Zernike polynomial in rectangular aperture.