The invention discloses a seawater desalting device for tail gas of an engine. The device comprises a superconductive pipe heat exchanger(4), a vacuum evaporating pot(6) and a condenser(5) which are orderly connected as a whole, wherein the superconductive pipe heat exchanger(4) is communicated with an exhaust pipe(2) of the engine through a flue gas diversion valve(3), and the condenser(5) has a seawater inlet provided with a sea water replenishing pump(10) and a condensed water outlet provided with a vacuum pump(9) and a fresh water collection pot(8). The invention utilizes the tail gas of the engine which is the waste energy to produce fresh water by the distillation method, which fully utilizes the energy, reduces the emission of pollutants and fully utilizes the heat energy of the tail gas; the seawater desalting device has the advantages of simple running, easy operation and maintenance, low cost, small volume and few requirements of the power section, and can effectively resolve the water supply problem for sailing warships, civilian ships and oil drilling platforms which use engines as power sources or use other devices the temperature of the tail gases of which is more than 80 DEG C, as power sources.