The invention discloses a flue gas flow channel used in a biomass fired boiler. The flue gas flow channel comprises a furnace arranged in the boiler, a convectional heating surface arranged on rear side of the boiler, a tail heating surface arranged outside the boiler and a flue gas outlet communicated with the tail heating surface. The flue gas flow channel further comprises a cooling sedimentation chamber arranged between the furnace and the convectional heating surface to shape the flue gas flow channel into a four-backstroke tortuous shape, that is, the furnace is communicated with the cooling sedimentation chamber through an outlet at upper part of the rear side thereof, the cooling sedimentation chamber is communicated with the convectional heating surface through an outlet at lower part of the rear side thereof, upper part of the rear side of the convectional heating surface is communicated with the tail heating surface by pipelines, and the flue gas outlet is connected to the lower part at the rear side of the tail heating surface. The flue gas flow channel used in a biomass fired boiler is added with a cooling sedimentation chamber between the furnace and the convectional heating surface based on the prior art to shape the flue gas flow channel into a four-backstroke tortuous shape, thus reducing soot deposit on the convectional heating surface, and enhancing heat exchange effect of the convectional heating surface.