The invention discloses a flue gas waste heat utilization system. The flue gas waste heat utilization system comprises a high-temperature flue gas cooler connected with an air preheater, wherein the high-temperature flue gas cooler is sequentially connected with a dust remover, a low-temperature flue-gas cooler and a desulphurization reactor; a heating medium channel of the high-temperature flue gas cooler and a heating medium channel of the low-temperature flue gas cooler are connected with a heating medium channel of the heat exchange device; the heating medium channel of the heat exchange device is used for heating air which is fed into the air preheater; an air high-temperature heater for cooling flue gas is arranged between a flue gas inlet of the air preheater and a flue gas outlet of a denitrification reactor; and an air outlet of the air preheater is connected with the air high-temperature heater. According to the system disclosed by the invention, utilization efficiency of boiler flue gas waste heat is greatly improved; operation safety of the air preheater is improved by regulating the temperature of the outlet flue gas of the air preheater; and meanwhile, the flue gas temperature at an inlet of the denitrification reactor at a low-load stage of a boiler also can be increased, and a load applicable range of the denitrification system is expanded.