The invention discloses a device for pretreating desulfurization wastewater by flue gas softening. The device comprises a wastewater aeration settling tank, a pH regulating tank, a blower and an automatic control device, wherein an aeration device A and an online pH monitor A are arranged in the wastewater aeration settling tank; a sludge drain outlet is formed in the bottom of the wastewater aeration settling tank; an aeration device B and an online pH monitor B are arranged in the pH regulating tank; the blower is connected with the wastewater aeration settling tank, the pH regulating tank and a flue gas heat exchanger; the automatic control device is connected with the online pH monitor A, the online pH monitor B, valves on pipelines and the blower and is used for controlling on and off of the valves and the blower according to the monitoring results of the online pH monitor A and the online pH monitor B. The device has the beneficial effects that CO2 in the flue gases is effectively utilized, the waste heat of the flue gases is reduced, and heat pollution of the power plant flue gases is reduced while carbon emission is reduced; evaporation of plenty of water and droplet entrainment caused due to high temperature of the flue gases in the flue gas aeration softening process are effectively reduced by pre-cooling the flue gases for softening; the equipment cost is reduced and the safety is improved.