The invention relates to a bottom sealing device of a band steel aerosol cooling device, which comprises an air knife (1), an air knife cylinder (2), an air inlet (3), a stretching mechanism (4), a band steel inlet (5), a water collecting sump (7), and a watertight shuttle (8). The water collecting sump (7) is a square ring shaped sump which comprises an inclined bottom plate (70), a water outlet (71), an inner square plate (72), a outer plate (73), a bottom plate (74) and an inner arc-shaped plate (75); and the nozzle of the air knife is a ring-shaped nozzle which comprises an upper air nozzle (11), a lower air nozzle (12), a lower arc-shaped air nozzle (13), a side air nozzle (14) and an upper arc-shaped air nozzle (15). The bottom sealing device of the band steel aerosol cooling device has the advantages of good sealing effect, simple equipment and stable working, can reduce equipment maintenance load, has high promotion value in the field of quick aerosol cooling after plating, and can be applied to hot galvanizing, aluminum and zinc hot dipping and zinc layer alloying production lines, and also be suitable to other occasions needing aerosol cooling.