A liquid-crystal spatial light modulator (LC-SLM) can be used to deflect laser beam, however its pixel structure and diffraction effect resulted from phase wrapping technique decreases its optical efficiency. The diffraction effect was analyzed theoretically and experimentally, and the relationship between fill factor and diffraction efficiency was demonstrated. 256 pixel×256 pixel liquid crystal spatial light modulator was used to alter the wavefront phase to achieve different deflections of a laser beam dynamically. The far field intensity distribution was measured at different deflection angles. The results show the diffraction efficiency decreases with the fill factor. Its measured diffraction efficiency was only 51.3% when the fill factor is 0.85. The phase wrapping technique make the LC-SLM form the structure asablazed grating and its diffraction efficiency decreases with the increase of the deflection angle.