A method for producing synthetic gas from biomass by: a) grinding the biomass, feeding the biomass into a pyrolysis furnace while spraying a first superheated water vapor into the pyrolysis furnace, controlling the temperature of the pyrolysis furnace at 500-800° C., contacting the biomass with the first superheated water vapor for a pyrolysis reaction to yield crude synthetic gas and ash including coke; b) cooling the ash, and separating the coke from the ash; c) transporting the crude synthetic gas and the coke into a gasifier, spraying a second superheated water vapor into the gasifier, controlling the gasifier at an operating temperature of 1200-1600° C., contacting the biomass with the second superheated water vapor for a gasification reaction to yield primary synthetic gas; and d) cooling, removing dust, deacidifying, and desiccating the primary synthetic gas to obtain clean synthetic gas.