The invention discloses a biomass fuel boiler and belongs to the technical field of boilers. The biomass fuel boiler disclosed by the invention mainly comprises a biomass inlet, a valve, a secondary air shunting pipeline, a secondary air pipeline, a slag discharge opening, an ash residue chamber, a vaporizing chamber, a grid plate, a solid material return pipe, a cyclone separator, a gasified gas collection pipeline, a premix burner, a combustion chamber furnace wall, a combustion chamber, a secondary high-temperature flue gas channel, a high-temperature flue gas chamber, a gasifying chamber and a secondary air chamber, wherein the grid plate is arranged at the bottom of the gasifying chamber, a baffle is arranged in the secondary air chamber. According to the biomass fuel boiler, biomass is gasified by virtue of intermediate temperature, and combustible gas generated through gasification can be combusted to release heat at a combustion section by virtue of a high-temperature oxygen-deficient combustion method, so that the combustion efficiency and the thermal efficiency are increased, the contradiction that chlor-alkali corrosion and slag-bonding are caused by high combustion temperature, tar generation and low energy utilization are caused due to low combustion temperature during the current biomass energy utilization process can be solved, and the high-efficiency, low-pollution and high-grade utilization of renewable energy sources can be realized.