The invention discloses a braze welding clamp of a plate-fin type heat exchanger. The clamp consists of a nut, a top bolt, a side top bar, a spring, an upper cover plate, a plate-fin structure, a lower bottom plate, a vertical bolt, a supporting plate, a measuring rod, a pressure plate, a middle top bar, a side guide sheet, a middle guide sheet, a bolt, a rolling ball and a pin, wherein the pressure plate is additionally arranged between the side top bar and the middle top bar; the measuring rod is inserted into the pressure plate in a through hole; the spring is placed between the supportingplate and the pressure plate; the supporting plate is placed on the upper cover plate; the plate-fin structure is positioned between the upper cover plate and the lower bottom plate; a part among thepressure plate, the upper cover plate and the lower bottom plate is of a hollow-out structure; the side top bar is assembled with the middle top bar through matching of a bolt and a nut; and each topbar is assembled with the lower bottom plate through matching of a bolt and a nut. The braze welding clamp can apply uniform pressure to the plate-fin structure in a vacuum braze welding process, so that distance among welding seams is reduced, a welding rate is increased, welding defects are reduced, and strength and plasticity of a braze welding joint are improved; and moreover, the clamp is high in heat transfer efficiency, is convenient in dismounting, and can shorten a braze welding period.