A chemical bath deposition method is presented to prepare different thin films on plane substrates. In particular, they are useful to deposit CdS or ZnS buffer layers in manufacture of thin film solar cells. This method and the deposition apparatus deposit thin films onto vertically travelling plane workpieces delivered by a conveyor belt. The thin films are deposited by continuously spraying the reaction solutions from their freshly mixed styles to gradually aged forms until the designed thickness is obtained. The substrates and the solutions are heated to a reaction temperature. During the deposition processes, the front surfaces of the substrates are totally covered with the sprayed solutions but the substrate backsides are remained dry. The reaction ambience inside the reactor can be isolated from the outside atmosphere. The method is designed to generate a minimum amount of waste solutions for chemical treatments.