The invention discloses a biomass gasification unit which comprises a hay cutter (1), a closed conveyor (2), a gasification furnace (3), a purifier (4), an oil-gas separator (5), a Roots fan (6), a pressure adjusting bypass (7), a cyclone tar collecting device (8), a backflow pipe (9), a safe water seal (10), a wet gas holder (11), an emptying pipe (12), a flame arrester (13) and a user air supply pipe (14). The biomass gasification unit has the beneficial effects of running under a complete closed state during the processes from the preliminary processing of biomass to be used by the user, so that the environment pollution is avoided, due to layer-upon-layer purification, the gas purity is higher, the tar content is reduced to minimum, and the burning heat value approaches to a straw theoretic calorific value; and the biomass gasification unit is simple and convenient in operation, safe, environment-friendly, economic, energy-conservation and high-efficiency, and is suitable for concentrated air supply for multiple users.