In order to calibrate the liquid crystal spatial light modulator (LCSLM) with high precision and meet the application requirements of linear phase modulation, a common-path heterodyne interferometer system was proposed to measure the phase modulation characteristics of LCSLM. The measurement principle was analyzed. Then the experimental data of the phase modulation varying with the input gray value of LCSLM was obtained and the linear calibration was carried out. The measurement results of the LCSLM show that the maximum phase modulation of the LCSLM used in the experiment is 2.55π. The phase modulation curve in the range of 20~240 is linearly corrected using the inverse interpolation method, and the corrected theoretical phase modulation curve is very close to the ideal linear curve. The correlation coefficient between the measured phase modulation curve and the ideal linear modulation curve can reach 0.9996. The proposed method can overcome the dependence of traditional measurement methods on image processing. The measurement precision of the proposed method is higher, and the phase modulation valves of LCSLM can be obtained directly through the lock-in amplifier. This research can provide a reference for high-precision wavefront correction and measurement based on LCSLM.