The invention discloses a waste heat recovery system and method with discharged smoke waste heat as a heat source of an air heater. According to the waste heat recovery system and method with the discharged smoke waste heat as the heat source of the air heater, a low-temperature economizer and the air heater are connected through a pipeline to form a first circulation loop, a circulating pump is arranged on an inlet pipeline of the low-temperature economizer, and an air heater intake water regulating valve is arranged on an inlet pipeline of the air heater; and the low-temperature economizer is connected with the circulating water side of a condensed water heater through a pipeline to form a second circulation loop, the condensed water side of the condensed water heater is connected with a low-pressure heater in parallel through a pipeline, and a booster pump is arranged on a condensed water connecting pipeline between the condensed water side of the condensed water heater and the low-pressure heater. The system and method have the advantages that the air heater is reserved, low-temperature corrosion of an air preheater is relieved, the low-temperature economizer recovers discharged smoke waste heat of a boiler, and power generation coal consumption of a steam turbine is reduced; and meanwhile, the defects that since the air heater consumes low-pressure suction steam of the steam turbine, the heat consumption of the steam turbine is increased, the temperature of discharged smoke is increased, and the heat efficiency of the boiler is reduced are overcome, and the problems that a low-temperature economizer pipeline system is high in cost and operating reliability requirement are solved.