Flue gas afterheat recovery system

The invention provides a flue gas afterheat recovery system comprising a phase-change exchanger and an automatic control device which is mounted on the phase-change exchanger. The phase-change exchanger comprises a phase-change upper section and a phase-change lower section which are communicated; the phase-change upper section of the phase-change exchanger is communicated with a main condensate pipeline in a low-pressure regenerative heating system; the phase-change lower section of the phase-change exchanger is arranged in a flue gas channel; a wall temperature tester is mounted on the phase-change lower section; a signal of the wall temperature tester controls the flow of condensate entering the low-pressure regenerative heating system of the phase-change upper section of the phase-change exchanger by the automatic control device; the condensate in the automatic control device is heated after entering the phase-change upper section of the phase-change exchanger, and the heated condensate enters the main co