The invention discloses a thin film clamping device. The thin film clamping device comprises a film clamping bracket, wherein a film clamping arm lifting mechanism and a film clamping arm horizontal clamping mechanism are arranged on the film clamping bracket; the film clamping arm lifting mechanism drives the film clamping arm horizontal clamping mechanism to descend and ascend; the film clamping arm horizontal clamping mechanism comprises two clamping arms; clamping fingers are arranged on the clamping arms; the two clamping arms of the film clamping arm horizontal clamping mechanism horizontally act; the clamping fingers of the two clamping arms are matched to clamp a thin film. The thin film clamping device of a battery box coating machine provided by the invention is simple in structure, accurate in action and reliable in operation.