The invention discloses a sub-control phase-change heat exchange system based on grading combination heat exchange. The sub-control phase-change heat exchange system comprises a flue, an air channel, an air pre-heater, 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 the waste heat of recycled flue gas are arranged in a grading combination mode; according to different situations such as ambient air temperature, operating conditions 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 combination mode; meanwhile, the temperature of the condensed water is improved through grading heating, higher energy quality is obtained, and the optimal economic benefit of recycling the waste heat is achieved.