A new PZT/a-Si:H composite thin film is introduced in this paper. Its whole structure is pt/PZT/CR/a-Si:H/ITO. PZT thin film was prepared by RF sputtering method in the thickness of 0.5-1.0 micrometers , while a- Si:H thin film was prepared by RF glow discharge method with its thickness controlled to match the PZT thin film in resistance. The resistivity of a-Si:H thin film will drop about four order under optical exposure. Hence, when the composite thin film is exposed to light, the voltage applied to it will be transferred from a-Si:H to PZT. This composite thin film has many advantages such as fast response, high density, cheap price and easy match with Si integrated circuit. A lot of new devices can be designed with this thin film: Photoelectric switch, photomemory, optical sensor, etc.