To calibrate liquid crystal spatial light modulators, a method for measuring their phase modulation characteristics is proposed based on the principle of polarization display. First, the modulation characteristics of the liquid crystal spatial light modulator to the polarization state of the incident polarized light are analyzed. Then, a theoretical relationship is established between the long axis direction of the elliptically polarized light after passing through the azimuthally polarization axis finder and the phase modulation amount of the liquid crystal spatial light modulator. By building an optical system, the phase characteristics of the liquid crystal modulator are measured. The results show that the maximum phase modulation of the spatial light modulator used is 2.45π rad, and the phase modulation curve within the gray scale range of 15 to 210 approximately meets the linear distribution. Finally, the phase modulation curve is linearly corrected by applying inverse interpolation, and a look-up table of the input gray scale and the driving gray scale is generated. The correlation coefficient between the corrected phase modulation curve and the ideal linear modulation curve reached 0.9993.