This paper proposes a new interferometric method to measure the phase modulation characteristics of a liquid crystal spatial light modulator (LC-SLM). In our proposed method, the beam was incident perpendicular to the SLM and the combined grayscale pattern loaded into the SLM consisted of three parts. The left part was a blazed diffraction grating, and the two right parts were grayscale with different constant values. The gray value in the downward section remained constant at zero, while it gradually increased from 0 to 255 in the upward section. By changing the gray value of the combined grayscale loaded on the LC-SLM, different sheared fringe patterns, generated by the interference between the constant phase-modulated beam and the +1 order diffracted beam of the blazed grating, could be obtained. The phase modulation value could then be calculated using only one sheared fringe pattern. As a result, our proposed method can reduce the effect of environmental vibration or air turbulence and improve measurement precision. The experimental results are presented to validate the method's potential.