The invention relates to an auxiliary combustion system integrated with a vertical shaft flue on the back of a pulverized coal boiler. The auxiliary combustion system comprises a boiler hearth, a horizontal flue, a vertical shaft flue and a pyrolysis furnace body, wherein the pyrolysis furnace body is arranged on a connecting part between the horizontal flue and of the vertical shaft flue and heated by flue gas; the upper end of the pyrolysis furnace body communicates with a feeding chamber, the lower end of the pyrolysis furnace body communicates with a charcoal collecting bin, the middle area of the pyrolysis furnace body communicates with a tar gas condensation bin, and the side wall of the upper end of the pyrolysis furnace body communicates with a combustible gas collecting bin through a steam separator; a spiral propelling rod which gradually increases in radial size from top to bottom is arranged in the pyrolysis furnace body in the axial direction; the hearth is provided with acombustor nozzle, a combustion stabilizing nozzle and a reducing nozzle; and the biological carbon collecting bin is connected with a burner nozzle through a coal mill, and the combustible gas collecting bin communicates with the combustion stabilizing nozzle and the reducing nozzle. The auxiliary combustion system has the advantages that the pyrolysis furnace body is directly arranged in the flue, so that the waste heat utilization efficiency is high; and the combustible gas and the biochar obtained by biomass pyrolysis can be used for effectively assisting the boiler for burning or being used for other purposes.