This paper gives a brief description of regener ative flue gas in FCCU. It disccuses the design feature of waste heat bo i-l er used fo r recovering heat ener gy of gas. That spira-l finned tube has a more compact structure. The deposition, low temperature corrosion and vibr atio n in the heat- exchanger surface should be considered so as to guarantee that the boiler functio ns pr operly .