The invention discloses a double-furnace type biomass boiler. The double-furnace type biomass boiler comprises a main furnace and an auxiliary furnace, wherein the middle parts of the main furnace and the auxiliary furnace are connected by a hot air pipe; the lower parts of the main furnace and the auxiliary furnace are connected by a water pipe; the main furnace comprises a main shell, a main combustion chamber and a main water storage chamber; the lower part of the main combustion chamber is provided with a fire grate; the fire grate comprises a round support pipe fit with the inner wall of the main combustion chamber, a plurality of rib pipes and a plurality of vertical air blowing pipes; the round support pipe is communicated with the rib pipes and the vertical air blowing pipes respectively; the rib pipes are provided with a plurality of upper air exhaust holes; the vertical air blowing pipes are provided with a plurality of radial air exhaust holes; the round support pipe is communicated with an air outlet of a fan via a connecting pipeline; the side surface of the main shell is provided with a combustion charge pipe; one end of the combustion charge pipe is communicated with the main combustion chamber, and the other end of the combustion charge pipe is communicated with a packing auger; an inlet of the packing auger is communicated with a fuel charge hopper. According to the biomass boiler, due to the design of double rows of furnaces, the height space is saved, so that the whole structure is miniaturized; by changing the structure of the fire grate and the air supplying mode of combustion-supporting air, fuels are combusted sufficiently, and the combustion efficiency is improved.