A method for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification, the method using a superheated water vapor as an oxidant and an energy carrier, conducting biomass pyrolysis and gasification at different temperature ranges, and finally producing clean synthetic gas; and the method comprising the following steps: a) grinding the biomass, feeding the biomass into a pyrolysis furnace while spraying a low temperature superheated water vapor, which is at a temperature of 500-800 °C and is produced by a low temperature plasma torch heater into the pyrolysis furnace, controlling the pyrolysis furnace at an operating temperature of 500-650 °C and controlling an operating pressure of the pyrolysis furnace at 105-109 kPa, contacting the biomass with the low temperature superheated water vapor for a pyrolysis reaction to yield crude synthetic gas and an ash comprising a coke; wherein an input speed of the low temperature superheated water vapor into the pyrolysis furnace is 35-50 m/s, a retention time of the crude synthetic gas in the pyrolysis furnace is 15-20 s, and an output speed of the crude synthetic gas from the pyrolysis furnace is 15-20 m/s, 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 high temperature superheated water vapor, which is at a temperature of 1200-1600 °C and is produced by a high temperature plasma torch heater into the gasifier, controlling the gasifier at an operating temperature of 1200-1600°C, contacting the crude synthetic gas and the coke with the high temperature superheated water vapor for conducting a gasification reaction to yield primary synthetic gas; and d) cooling, dust removing, deacidifying, and desiccating the primary synthetic gas to obtain clean synthetic gas. a) grinding the biomass, feeding the biomass into a pyrolysis furnace while spraying a low temperature superheated water vapor, which is at a temperature of 500-800 °C and is produced by a low temperature plasma torch heater into the pyrolysis furnace, controlling the pyrolysis furnace at an operating temperature of 500-650 °C and controlling an operating pressure of the pyrolysis furnace at 105-109 kPa, contacting the biomass with the low temperature superheated water vapor for a pyrolysis reaction to yield crude synthetic gas and an ash comprising a coke; wherein an input speed of the low temperature superheated water vapor into the pyrolysis furnace is 35-50 m/s, a retention time of the crude synthetic gas in the pyrolysis furnace is 15-20 s, and an output speed of the crude synthetic gas from the pyrolysis furnace is 15-20 m/s, 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 high temperature superheated water vapor, which is at a temperature of 1200-1600 °C and is produced by a high temperature plasma torch heater into the gasifier, controlling the gasifier at an operating temperature of 1200-1600°C, contacting the crude synthetic gas and the coke with the high temperature superheated water vapor for conducting a gasification reaction to yield primary synthetic gas; and d) cooling, dust removing, deacidifying, and desiccating the primary synthetic gas to obtain clean synthetic gas.