A liquid crystal spatial light modulator (LC SLM) with parallel alignment TFT is prepared, the theory of which is also discussed. And properties of the modulation are theoretically calculated. Measurements are conducted on properties of the phase modulation and the amptitude modulation. The results indicate that it is a pure phase modulator in the range of the gray level. Its precision is measured on a ZYGO interferometer. The experiments show that on the area of 1 cm×1 cm the peak to valley (PV) value is up to 0.098λ, RMS up to 0.017λ, and the depth of modulation up to 0.6λ after correction. A grating configuration with the phase differences of π/2 is accordingly generated, which shows that the phase of light can be medulated with it quite well.