A self-interference method for measuring phase modulation characteristics of parallel liquid crystal spatial light modulator (SLM) is proposed. The method is experimentally simple and robust. The gray image loaded onto the SLM is composed of three parts, one is a binary diffraction grating grayscale map, the other two maps are codified by a uniformly distributed gray level area, where the gray value below the part is constantly 0, the upper part gray value is gradually increased from 0. With the gray value of the upper part changing, the reflected beam by modulation interferes with the +1 order grating diffraction wave to generate a whole dislocation fringes, the amount of SLM phase modulation can be calculated by measuring the cycle and the relative displacement between the upper and lower fringes on the same interference pattern. This proposed method eliminates the influence of the fringe jitter caused by ambient vibration or air turbulence on the measurement and improves the measurement accuracy. Compared with other methods, the new method which obtains the phase modulation characteristics of SLM does not require complex optical devices, and has excellent mechanical stability, fast response and easy to implement.