According to the dynamic response and phase modulation of the liquid crystal,the liquid crystal display (LCD) used for optical testing was investigated in the paper. The LCD was reconstructed into a phase-only spatial light modulator in the experiment. And the phase modulating characteristics were measured. It shows the phase modulation depth is more than one wavelength. The sphere wave with phase modulation magnitude 3.4 λ is achieved by the kinoform method. The former surface of a convex lens was tested by ZYGO interferometer. The parallel straight fringes are obtained and PV is 1/3 λ.