To solve the high precision of testing of aspheric surface or even free-form surface, the line binary grating theory is used, gave the influence result of each errors of the computer generated hologram (CGH), based on an 846 mm×630 mm free-form surface, the CGH to test the surface is designed and manufactured. The result shows that when design CGH to test surface, first separate the diffraction order, change the distance between the focus of the interferometer to CGH to decrease the pattern distortion; calibrate the substrate errors; at last choose duty-cycle to 0.5, etching depth to 0.3λ to decrease duty-cycle errors，phase function errors and amplitude errors when encoding.