The invention discloses a biomass gasifier, comprising a gasifier body, a grate, an ash chamber, a support for supporting the gasifier and an ash discharging mechanism, wherein, the grate is arrangedin the gasifier body; the ash chamber is positioned below the gasifier body; the grate is in a flat structure with no hole in the middle; an ash discharging hole is arranged at the edge part at whichthe inner walls of the grate is connected with the gasifier body; an ash poking mechanism used for poking the ash residue on the grate into the ash discharging hole is additionally arranged on the grate; and the ash poking mechanism is connected with an ash poking drive mechanism which is arranged in a shield positioned below of the grate. By adopting the ash poking drive mechanism to drive the ash poking mechanism, the invention can realize the purpose of continuous ash discharge of ash residue on the grate, is beneficial for successfully carrying out the chemical reaction of biomass raw materials to improve the work efficiency of the gasifier, the middle part of the gasifier is in a flat structure with no hole, thereby avoiding the defect of materials leakage of the existing grate; and the shield arranged the ash chamber below the grate is used for protecting the ash poking drive mechanism.