Starting from the sampling theorem,the fast Fourier transform (FFT) computation of angular spectrum diffraction formula and its inverse operation are studied. Based on the studied results,the wavefront reconstruction problem in digital holography,that is,the object light passing through an optical system and then reaching CCD detector,is studied. Two methods are proposed: inverse operation tracking reconstruction of the object wave field and wavefront reconstruction in image space. Finally,the feasibility of these two methods is proved by digital holography experiments and the calculation of wavefront reconstruction.