The invention discloses a system for reducing water vapor emission from a quenching slag bath during water quenching of yellow phosphorus slag, and relates to the technical field of water quenching ofyellow phosphorus slag. The system comprises floating balls, a gas collecting hood, a fan, an exhaust chimney and a sprayer. The gas collecting hood is disposed directly above a slag dropping port which is located above the quenching slag bath, and a number of float balls are arranged on the water surface of the quenching slag bath. The top of the gas collecting hood is in communication with theexhaust chimney, and the sprayer and the fan are sequentially arranged in the gas collecting hood. The sprayer is connected to a water reservoir through a water supply pipe, and the fan is connected to a power source through a wire. The floating balls are placed with the sides of adjacent floating balls being closely against each other over the water surface, reducing the evaporation surface of the water and thereby suppressing the evaporation of the water; the water vapor near the slag dropping port is introduced into the exhaust chimney by means of the fan through the gas collecting hood, and thus emitted intensively; a part of the water vapor is cooled into water by the spray condensation of the sprayer, and then returned to the quenching slag bath for reuse, which greatly reduces the emission of atmospheric pollutants and saves water resources.