The invention discloses a biomass combustion furnace. The biomass combustion furnace comprises a surplus smoke circulation device and a surplus smoke purification device. The surplus smoke circulation device comprises a hot air pipe, a circulation cavity and a return air pipe. The inlet end of the hot air pipe communicates with a heating area at the top of a hearth, and the outlet end of the hot air pipe communicates with the circulation cavity. The horizontal height of the circulation cavity is larger than that of the top of the hearth. The inlet end of the return air pipe communicates with the circulation cavity, and the outlet end of the return air pipe communicates with the bottom of the hearth. The surplus smoke purification device comprises a purification cavity and a first evaporation disc. The purification cavity is formed below the circulation cavity and communicates with the circulation cavity, and an exhaust opening is formed in the bottom of the purification cavity. The top of the first evaporation disc is open, the first evaporation disc is arranged in the purification cavity, and evaporation liquid is arranged in the first evaporation disc.