The invention relates to a flue gas waste heat utilizing system of a boiler, and particularly discloses a flue gas waste heat utilizing system of a condensed water heating boiler for air supply. The flue gas waste heat utilizing system comprises a steam turbine condensed water pipeline and is characterized in that a high-temperature flue gas heat exchanger is arranged in a front vertical flue of an electrostatic dust collector, a low-temperature flue gas heat exchanger is arranged in an inlet flue of a desulfurizing tower, the high-temperature flue gas heat exchanger communicates with the condensed water pipeline through an inlet pipeline, and return water communicates with the condensed water pipeline through an outlet pipeline; the low-temperature flue gas heat exchanger communicates with the condensed water pipeline through an inlet pipeline, outlet water respectively enters a primary air heater and a secondary air heater through water inlet pipelines of the air heaters, and return water enters the condensed water pipeline through water return pipelines of the air heaters. According to the flue gas waste heat utilizing system, the flue gas temperature can be effectively reduced to about 85 DEG C, and the overall operation benefit of a power plant is increased by utilizing flue gas waste heat; in addition, the flue gas temperature of an inlet of the electrostatic dust collector is reduced, so that the specific resistance of dust is effectively reduced, and the dust collection efficiency of the electrostatic dust collector is improved.