Abstract: This paper introduces the basic structure and light wave modulation principle of liquid crystal on silicon(LCOS) device.When the polarization direction of light wave is accordant with the optic axisof liquid crystal polymer, LCOS is under phase-only modulation model.This paper also introduces the principle of Zygo interferometer phase-difference measurement, measures the phase modulation characteristics with American Zygo interferometer of RL-SLM-R2 reflection pure-phase LCOS produced by Beijing Reallight Technology Co., Ltd, revises the measuring results according to the interferometer principle, and gets the accurate phase characteristic curve, of which the modulation range is 0-0.7154λ.This experiment creates a new method for measuring phase modulation, which can help better utilize LCOS.