The slow response has limited further application of phase only liquid crystal spatial light modulator in many research fields, such as adaptive optics and information optics. Polymer network liquid crystal (PNLC) is proposed to apply in phase only liquid crystal spatial light modulator to enhance the response speed to sub-millisecond time scale. The light scattering and transient response on applied voltage are elucidated. The light scattering of PNLC is minimized by lowering the curing temperature and increasing the curing intensity. The morphology of polymer network is also taken by scanning electron microscope (SEM) to elucidate the mechanism of light scattering in polymer network liquid crystal. Upon loading large voltage to generate large phase retardation, electrostriction effect is found to slower the response time to the order of second scale. Lower threshold voltage is proposed to minimize the influence of electrostriction by employing large dielectric anisotropy liquid crystal solvent.