The utility model provides a buoyancy switch and hydrops alarm device, the buoyancy switch includes: casing, first, two contact parts, bodies, conductor part, the shaping has the trompil on the casingfor messenger's hydrops dipes, first, two contact parts are along vertical direction settings, and the top of casing is passed as first, two ends of buoyancy switch to one end, and the other end is set for highly with casing bottom interval, the distance is set for at first, two contact part intervals, body density is less than the density of hydrops, sets up in the bottom of casing, conductor part is fixed in the top of body, and when the hydrops diped the casing and makes the body rise, conductor part's both ends contacted with the other ends first, two contact parts respectively, make first, two ends of buoyancy switch switch on. The utility model provides a technical scheme can replace the manual work to patrol and examine, to a small amount of hydrops of low -lying lands such as cable pit go on in time, high -efficient, accurate warning, sensitivity is high, does not receive liquid electric conductive property's influence, but the wide application.