The invention discloses a positive-pressure type downdraft biomass gasifier. The positive-pressure type downdraft biomass gasifier comprises a furnace body, an inner furnace barrel, an ash discharge device and an air inlet tube, wherein the furnace body tightly covers the outer side of the inner furnace barrel, a feeding tube is arranged at the top of the inner furnace barrel, an airtight water steam chamber and a combustible gas chamber are formed in the upper part of the inner furnace barrel from top to bottom sequentially, a water inlet pipe and water steam pipes are arranged in the water steam chamber, a combustible gas outlet tube and gas making manifolds are arranged at the combustible gas chamber, the air inlet tube extends into the inner furnace barrel, an annular confined space formed by an annular groove is formed in each of two positions, located at the middle part and the middle-lower part of the inner furnace barrel, of the inner surface of the furnace body, exhaust holes communicated with the annular confined spaces are formed in the side wall of the inner furnace barrel, and the two annular confined spaces are communicated with the combustible gas outlet tube. The positive-pressure type downdraft biomass gasifier has the benefits as follows: water vapor is prepared from heat of the gasifier without an additional water vapor generation device, and the structure of the gasifier is simplified; multiple combustible gas output channels are arranged, so that combustible gas is timely, rapidly and completely output.