The invention discloses a device and a method for testing the water permeability of a planar filter medium for liquid and solid separation. The device mainly comprises a support baseplate, a baseplate support leg, a support pole, a water outlet, a lower connecting flange, a sealing ring, a supporting screen, a filtering element mounting plate, a fastening clamp, an upper connecting flange, a barrel, a water inlet, a water inlet sealing socket, an exhausting and pressurizing port, an incoming water distribution plate, an overflow port positioning rod, an overflow water outlet ring, an overflowoutlet pipe, an overflow port sealing socket, an overflow adjusting pipe, an overflow port elbow, a liquid level meter and the like. Due to the adoption of the test device and the method of the invention, the water permeability of the planar filter medium can be tested correctly and rapidly under a vacuum or pressurized condition and basis for scientific evaluation, selection and use of the filter medium is provided.