Duty cycle of computer-generated hologram（CGH）must be calculated to obtain an optimal contrast of the interference fringe pattern in testing a cubic phase plate.According to Scalar-diffraction theory,the relation between the contrast of the interference fringe pattern and duty cycle is researched.The calculation for the duty cycle of a Cr-glass reflective CGH is described in the paper.A CGH for testing a cubic phase plate with duty cycle 0.5 is fabricated by using e-beam lithographic techniques.In this situation,the contrast of the interference fringe pattern can achieve a level of 99.4% by calculation.