The invention relates to a device and method for testing permeability performance of filtering media in an unstable state. The device comprises an upper barrel (1), a liquid level dividing rule (2), an upper connecting flange (3), a barrel sealing gasket (4), a filtering sample mounting plate (6), a supporting screen plate (7), an O-shaped sealing ring (9), a lower connecting flange (10), a water outlet valve (11) and a fixed support. The device is characterized by comprising a fastening clamp (8) for connecting the upper connecting flange (3) and the lower connecting flange (10) in a fastening manner, wherein a sealing cavity body is formed between the upper connecting flange (3) and the lower connecting flange (10). Compared with prior art, the device and the method provided by the invention have the advantages of being capable of simply, rapidly and effectively determining the permeability performance of most flat plate filtering media so as to provide scientific bases for selection and use of the filtering media, and the like.