Gas-fired boiler afterheat recovering system and recovering method

The invention discloses a gas-fired boiler afterheat recovering system and a gas-fired boiler afterheat recovering method. A bypass flue is parallelly connected onto a main flue, a bypass flue butterfly valve, a boiler coal economizer, an air preheater and a condensing type heat exchanger are sequentially and serially connected onto the bypass flue, the bypass flue is communicated with a bypass chimney, a water inlet of a heat exchange pipe of the boiler coal economizer is connected with a water tank through a water supply pipeline, a water outlet of the heat exchange pipe is communicated with a boiler through a pipeline, an air inlet of a heat exchange pipe of the air preheater is communicated with a fan through a pipeline, an air outlet is communicated with a boiler combustion-supporting air inlet pipe through a pipeline, a water inlet of a heat exchange pipe of the condensing type heat exchanger is communicated with the water tank through a descending pipe, and a water outlet is communicated with the water