Computer-generated hologram (CGH) is more and more popularly applied in the high-precision testing of asphere as high-accuracy null compensator. The errors of design, coding, fabrication and alignment restrict the testing precision. A practicable method to test the asphere precisely is presented, and the calibration of CGH substrate, the calibration of reference surfaces, as well as distortion correction to improve the testing precision are accomplished. In order to obtain the measurement precision, in addition to the traditional root square sum (RSS) error analysis, the testing precision is directly evaluated through experimental comparison with the method of aberration free image conjugate point. The experimental result shows that the testing precision of asphere with CGH could reach 3.9 nm [root mean square (RMS)].