The invention discloses a method and a device for cooperative flue gas desulfurization and sewage organic matter degradation and denitrification. Flue gas is subjected to desulfuration and purification through a spraying tower, and desulfurized waste liquid and sewage flow into a sulfite reduction reactor to be subjected to anaerobic biological treatment, so that organic matters in the sewage are degraded, and meanwhile sulfite is reduced; then the sewage and a reflux liquid in a nitration reactor are mixed, and flow into a denitrification reactor, nitrate in the sewage is reduced into nitrogen gas to realize nitrogen element removal, and sulfide in the sewage is oxidized into sulfate or elemental sulfur in an oxygen deficiency manner to realize part desulfuration; then the sewage flows into the nitration reactor again, organic matters in the sewage is further degraded, meanwhile ammonia nitrogen is oxidized into nitrate, and is removed after flowing back into the denitrification reactor; the rest sulfide is completely oxidized into sulfate, and the sewage is purified, and can be directly discharged. The average removal rate of the organic matters in the sewage is 95%, the ammonia nitrogen removal rate is 90%, the total nitrogen removal rate is above 70%, and the sulfide removal rate is 99.9%. The flue gas-sewage cooperative treatment process capable of realizing efficient flue gas desulfuration, sewage decarbonization and denitrogenation at the same time is established, flue gas desulfurized waste liquid is treated, and sewage is purified.