Liquid crystal spatial light modulator(LC-SLM) has been widely used in many fields such as optical information processing,but it must to measure the phase modulation characteristics accurately.A new method for measuring the phase modulation characteristics of LC-SLM is proposed in this paper,in which the principle of holographic interferometry and the digital holographic zoom system are used.The phase modulation characteristics are calculated through two digital holograms,and at the same time the system error is eliminated,so the requirement of optical component adjustment precision is reduced,and the diffraction calculation is also omitted.The paper introduces associated measurement principle,presents the measurement process,calculation method and experimental results,and proves that the method has many advantages including simple system,low system error and fast measurement speed.