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