The invention discloses a biomass gasifier capable of cracking tar. The biomass gasifier comprises a vertically-arranged gasifier body, a secondary cracking chamber, an annular ventilation pipe, a plurality of gas inlet pipes and a gas guiding pipe, wherein the vertically-arranged gasifier body comprises a gasifying chamber, a combustion chamber and a gas collecting chamber; the secondary cracking chamber is arranged outside the combustion chamber in a surrounding manner, and is filled with a tar cracking catalyst; the annular ventilation pipe is arranged outside the secondary cracking chamber in the surrounding manner; the inner side wall of the annular ventilation pipe consists of one part of the outer side wall of the secondary cracking chamber; a plurality of gas holes are formed in the inner side wall of the annular ventilation pipe; the directions of hole ducts of every two adjacent gas holes are different; the annular ventilation pipe is communicated to a high-temperature air supplying device; one end of each gas inlet pipe extends into the combustion chamber along the tangential direction of the combustion chamber; the other end of each gas inlet pipe is communicated to a combustion-supporting gas supplying device arranged outside the gasifier body; one end of the gas guiding pipe is communicated to the gas collecting chamber, and the other end of the gas guiding pipe is communicated with the secondary cracking chamber. According to the biomass gasifier disclosed by the invention, the content of tar in combustible gas is reduced, and the quality of the combustible gas is improved.