A chemical bath deposition method and an apparatus are presented to prepare 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 system deposit thin films onto vertically travelling plane substrates delivered by a conveyor belt. The thin films are deposited with 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 a deposition process, front surfaces of the substrates are totally covered with the sprayed solutions but their backsides are remained dry. The reaction ambience inside the reactor can be isolated from the outside atmosphere. The apparatus is designed to generate a minimum amount of waste solutions for chemical treatments.