A liquid crystal spatial light modulator (LCSLM) has a wide range of applications in the non-display field. In this study, the phase modulation characteristics of LCSLM are evaluated using a Shack-Hartmann wavefront detector. The results denote that the linear phase modulation range of the low-order Zernike mode is relatively large, and the diffraction efficiency significantly decreases as the modulation range increases. Furthermore, the possibility of using LCSLM for performing aspherical surface measurements is discussed, which provides a new approach for expanding the method of aspheric surface shape detection.