The invention belongs to the technical field of atmosphere contaminant control, and particularly relates to a system and method for ultralow discharge of boiler flue gas. The system comprises a hearth, a coal economizer, an air preheater, a heat exchanger, a dust remover, a desulfurizing tower, an induced draft fan, a cleaning device, a denigration reactor and a chimney. The method comprises the steps that boiler flus gas enters the dust remover through the coal economizer, the air preheater and the heat exchanger, after dust removal is performed, the flue gas enters a desulfurizing tower, after desulfurization, the flue gas is fed into the cleaning device through the induced draft fan, after the flue gas is cleaned, the flue gas is heated through the heat exchanger and then enters the denigration reactor, and finally, the flue gas is exhausted through the chimney. According to the system and method for ultralow discharge of boiler flue gas, the ultralow discharge requirements that thesmoke concentration is not larger than 5 mg / Nm<3>, the SO2 concentration is not larger than 35 mg / Nm<3>, and the NOx concentration is not larger than 50 mg / Nm<3> are met.