To specify the large aperture flat mirror, the discrete Fourier transform (DFT) is discussed in the processing for the slope data achieved from the pentaprism. The basic mathematic principle is processed to analysis the advance of DFT in processing the slope data. Then, the Zernike polynomials and power spectral density method are involved to evaluate the spatial domain and spatial frequency domain property of the method involved in this paper. For the large reflecting mirror, the algorithm is verified to make sure that it is enough to specify the low order mirror figure.