The invention discloses a flue gas residual heat boiler, and belongs to energy-saving and residual-heat recovery equipment. The flue gas residual heat boiler consists of a steam pocket, a descending pipe, ascending pipes and a residual heat recoverer, wherein softened water enters the steam pocket through a steam pocket water inlet, and then enters headers along the descending pipe; the water is uniformly distributed to heat exchange pipe sets through the headers, and besides, flue gas passes from one side to the other side of each heat exchange pipe set, so that the water in each heat exchange pipe set is heated and converted into steam; then, the steam enters headers at the other end along heat exchange pipes, enters the steam pocket through the ascending pipes, and is filtered through a steam-water separator; and finally, the steam flows out from a steam outlet. The whole flue gas residual heat boiler is simple in structure, convenient to use, long in service life and high in heat transferring efficiency, and is applicable to the recovery of high-temperature flue gas generated by large-size boiler bodies in industrial production.