The invention discloses a gasification agent injection device of a biomass gasifier, which comprises a gasification agent inlet pipe, a gasification agent outlet pipe erected in a gasifier body, an electric or manual speed reducer positioned at the top of the gasifier and an air inlet three-way joint fixed in the gasifier body, wherein the upper part of the gasification agent outlet pipe runs through upper and lower ports of the three-way joint to be connected with a main shaft of the speed reducer and to rotate along with the main shaft of the speed reducer under the positioning of the gas inlet three-way joint; the part of the gasification agent outlet pipe in the three-way joint is provided with a plurality of air inlets; the lower end of the gasification agent outlet pipe is communicated with at least one gasification agent injection pipe with a plurality of air injection holes; a transverse port of the air inlet three-way joint is connected with the gasification agent inlet pipe;joints between the gasification inlet pipe and the port of the air inlet three-way joint and between the gasification agent outlet pipe and the port of the air inlet three-way joint are in the state of high-temperature oil seal; and the outer pipe wall of the gasification gas outlet pipe is also provided with a material stirring device.