The invention provides a system and a method for treating sewage. The system comprises a thick grid, a water collecting tank, a fine grid, a rotational flow sand settling tank, a regulating tank, an emergency tank, a coagulation sedimentation tank, gas flotation equipment, an intermediate water tank, ultra-filtration equipment, a concentrated water regulating tank, a multistage coagulation sediment, an A2O biochemical tank, an inclined-tube sedimentation tank, a fiber turntable filter tank, a sludge concentrated water tank, a sludge dewatering room, a disinfection tank and a water recycling tank. The system and the method have the advantages that diversified processes for treating the sewage are combined with one another to form the complete system for treating the sewage, accordingly, different stages of sewage can be assuredly effectively treated, and treatment effects can be improved; outlet water treated by the aid of the system is high in quality, requirements of national emission indexes can be met, the outlet water can be applied to life again, and accordingly water resources can be reused.