In the paper,the phase modulation of parallel aligned LC wass discussed theoretically. The theoretical results were simulated with computer.Liquid crystal spatial light modulator(LCSLM) with parallel alignment TFT was prepared.And its properties of phase modulation were investigated.The results indicated that phase was controllable in the range of applied grey levels.The depth of modulation was up to 0.7 μm.And the precision was 0.052 μm.The experiment was conducted on the ZYGO interferometer,and good results were obtained.