A method for pyrolysis and gasification of biomass, the method employing a solid particle having a high thermal capacity as an energy carrier and saturated water vapor as an oxidant, conducting pyrolysis and gasification of the biomass in a pyrolysis furnace and a gasifier with inner cavities thereof interconnected, and finally producing clean synthetic gas; the method comprising the following steps: a) disposing the gasifier on the pyrolysis furnace, interconnecting the inner cavity of the gasifier and the inner cavity of the pyrolysis furnace; heating the solid particles, introducing the solid particle from an upper end of the gasifier into the gasifier and the pyrolysis furnace in sequence; controlling an operating temperature of the inner cavity of the pyrolysis furnace at 500-800°C and an operating temperature of the inner cavity of the gasifier at 1200-1600°C; b) grinding the biomass, feeding the biomass into the pyrolysis furnace while spraying the saturated water vapor into the pyrolysis furnace, contacting the biomass with the saturated water vapor at 500-800°C for pyrolyzing the biomass to yield crude synthetic gas and ash comprising coke; c) separating the ash comprising the coke from the solid particles, heating the solid particles, and transporting the solid particles into the gasifier for a next circulation; d) cooling down the ash, and separating the coke; e) introducing the crude synthetic gas into the gasifier via the interconnected inner cavities, transporting the coke into the gasifier while spraying the saturated water vapor into the gasifier, contacting the coke and the crude synthetic gas with the saturated water vapor at 1200-1600°C for gasifying the coke and the crude synthetic gas to yield primary synthetic gas; and f) cooling, removing dust, deacidifying, and desiccating the primary synthetic gas to yield clean synthetic gas. a) disposing the gasifier on the pyrolysis furnace, interconnecting the inner cavity of the gasifier and the inner cavity of the pyrolysis furnace; heating the solid particles, introducing the solid particle from an upper end of the gasifier into the gasifier and the pyrolysis furnace in sequence; controlling an operating temperature of the inner cavity of the pyrolysis furnace at 500-800°C and an operating temperature of the inner cavity of the gasifier at 1200-1600°C; b) grinding the biomass, feeding the biomass into the pyrolysis furnace while spraying the saturated water vapor into the pyrolysis furnace, contacting the biomass with the saturated water vapor at 500-800°C for pyrolyzing the biomass to yield crude synthetic gas and ash comprising coke; c) separating the ash comprising the coke from the solid particles, heating the solid particles, and transporting the solid particles into the gasifier for a next circulation; d) cooling down the ash, and separating the coke; e) introducing the crude synthetic gas into the gasifier via the interconnected inner cavities, transporting the coke into the gasifier while spraying the saturated water vapor into the gasifier, contacting the coke and the crude synthetic gas with the saturated water vapor at 1200-1600°C for gasifying the coke and the crude synthetic gas to yield primary synthetic gas; and f) cooling, removing dust, deacidifying, and desiccating the primary synthetic gas to yield clean synthetic gas.