As one of the most promising methods in all the interferometric null tests, computer-generated holograms (CGH) can also be used in unsymmetrical aspherical surfaces testing. Taking the cubic surface as an example, the principle of testing unsymmetrical aspherical surfaces with CGH is given. The expression of the aberration for the cubic phase during propagation is deduced, and the results of the optical design for the cubic surface test system are listed. The separation of diffraction orders and the binary process of the continuous CGH phase function are discussed in detail, and the sample of the amplitude CGH is given. The minimum line spacing of the CGH is 40 μm and its effect on wavefront error is only 0.005λ. The tolerance analysis of testing system is done in detail, and the total effect from all alignment tolerances that affect testing system is 83.954 nm (root-mean-square).