The pure phase modulation characteristics of a liquid crystal spatial light modulator are measured at oblique incidence using Zygo interferometer. The establishment of a mathematical model of pure phase modulation of liquid crystal spatial light modulator at oblique incidence, the effects of oblique incident angle of the phase modulation characteristics, structures based Zygo interferometer measurement system, and phase modulation characteristics under various incident angles are measured. The measurement results show that, at various incident angles, phase modulation increases approximately linearly, and the larger the incident angle, the smaller the increasing slope. With the same gray scale value the phase modulation amount decreases when the incident angle increases. When the incident angle is less than 5°, phase modulation curves almost coincide, and the maximum phase difference of the phase modulation amount is 0.032l with the same grayscale value.