The utility model discloses a sub-control phase-change heat exchange system based on grading combined heat exchange, which comprises a flue, an air channel, an air preheater, an external water pipeline, a steam supply valve of a low-temperature water heater, the low-temperature water heater, a water inlet valve of the low-temperature water heater, a steam-liquid heat exchanger, a steam supply valve of the steam-liquid heat exchanger, a steam supply valve of an air channel heat exchanger, an external steam supply valve of the air channel heat exchanger, a main water draining pipeline, the air channel heat exchanger, a water tank, a water pump, a water inlet valve of a flue heat exchanger, the flue heat exchanger, and a steam outlet valve of the flue heat exchanger. According to the sub-control phase-change heat exchange system, heat release exchangers which utilize waste heat of recycled flue gas are arranged in a grading combined mode; according to different situations such as ambient air temperature, operating and the like, the waste heat can be utilized to heat boiler supplied air, condensed water on the side of a steam turbine and the like in a combined mode; and additionally, the temperature of the condensed water is increased through grading heating, higher energy quality is obtained, and the optimal economic benefit of recycling the waste heat is achieved.