The invention discloses a floating passive cooling device of a spent fuel pool in a nuclear power plant. The passive cooling device at least comprises a heat exchange tube assembly. The heat exchange tube assembly comprises a plurality of heat exchange tubes and a grillwork, wherein the heat exchange tubes are filled with a certain quantity of cooling working media, and certain negative pressure is kept inside each heat exchange tube; the heat exchange tubes are evenly arranged on the grillwork in an equidistant mode, and the grillwork can generate a certain amount of buoyancy in liquid of the spent fuel pool so that the heat exchange tube assembly can float on the liquid level of the spent fuel pool. The portion, placed under the liquid level of the spent fuel pool, of each heat exchange tube is a heat absorption end, and the portion, placed over the liquid level of the spent fuel pool, of each heat exchange tube is a heat release end. The cooling working media are low boiling point working media. The invention further discloses a floating passive cooling system of the spent fuel pool in the nuclear power plant. By implementing the floating passive cooling device and system of the spent fuel pool in the nuclear power plant, operating can be conducted in a completely passive mode, so that safety is improved; the heat radiating area is further increased, and the heat removal capacity is greatly improved; the heat transfer working media can be recycled, and the device and system are environmentally friendly and stable.