The invention relates to the technical field of water softening apparatus, and relates to a flue gas hardness reducing apparatus and a use method thereof. The flue gas hardness reducing apparatus comprises a mixer, a fan and a reactor, the left lower end of the reactor is provided with a gas inlet chamber, the left upper end of the reactor is provided with a water inlet chamber, the right upper end of the reactor is provided with a gas outlet chamber, the right lower end of the reactor is provided with a water outlet chamber connected with the reactor, the gas inlet of the fan is provided with a flue gas conveying tube, the gas outlet of the fan is connected with the first inlet of the mixer through an air inlet tube, the second inlet of the mixer is connected with a medicine adding pipeline, and the outlet of the mixer is connected with the gas inlet chamber. The apparatus has a reasonable and compact structure, is convenient to use, and realizes no toxic side products, no large water property change, small generated mud residue quantity and small labor intensity of operators by using CO2 as a water softening agent; and CO2 in the flue gas is used as a gas source to realize the recycling of the flue gas and reduce the emission of CO2.