We report the fabrication and operation of an optically addressed spatial light modulator(OASLM), consisting of a hydrogenated amorphous silicon (α-Si: H) pin photodiode as a photosensor and a twist nematic liquid crystal(TNLC) as the modulating material. The intrinsic and doped α-Si: H films are deposited continuously by pECVD method in a reaction chamber with a shutter. The optical measurement indicates that the OASLM has a resolution of at least 20 lp/mm. The contrast ratio under the 300 lux illumination of white light is higher than 25:1. The white light writing sensitivity is about 3 lux.