The flue gas biological desulfurization system solves the problem of high operating costs and secondary pollution in existing flue gas desulfurization technologies. This technology cleverly utilizes the high pH and alkalinity of aerobic secondary sedimentation effluent from sewage treatment plants to spray and wash flue gas, resulting in high desulfurization efficiency and certain denitrification and dust removal effects. This technology has simple operation, low operating cost, stable and guaranteed efficiency, and is very suitable for flue gas desulfurization, denitrification, and dust removal.