According to Lohmann’s general theory on computer-holography, the principle of the computer-generated holopram of interference-fringe type is explained. An improvment upon the calculation procedure of Sirohi et al. has been made. It raises the accuracy and reduces the required time a little. A prototype hologram that reconstructs the required wavefront with an accuracy of 1/20 wavelength (rms.error) has been produced for testing a parabolic mirror (f=1021 mm, 2r0=214 mm), which shows the defects of the tested element.