Disclosed is a flood prevention device quick to mount and dismantle, relating to the technical field of waterproof baffles of building doors, in particular, relating to improvements in the installation and disassembly structure of waterproof baffles. The flood prevention device comprises a flood prevention baffle disposed at the inner edge of a door, wherein the width of the flood prevention baffle body is smaller than the width of the inner edge of the door; the middle of the flood prevention baffle is provided with a pair of lateral ejector rods, and the inner ends of the pair of ejector rods are provided with threads in different screwing directions; a double-headed nut is connected to the inner ends of the pair of ejector rods; and the outer ends of the pair of ejector rods realize horizontal movement under the action of the double-head nut. The width of the flood prevention baffle is slightly smaller than the width of the door. After being placed in position, the width of the flood prevention baffle is adjusted, so that moving pillars at two sides of the flood prevention baffle and the inner edge of the door achieve water sealing, and the bottom of the flood prevention baffleand the bottom surface of the door frame (the sill surface) achieve water sealing. The invention has the advantages of convenient operation, exquisite structure and high reliability, and can be widelyapplied to all places involved in waterproofing, and has great application prospects.