NOVELTY - An external heating-type microwave plasma gasification furnace, comprises cylindrical gasification furnace main body (2) vertically arranged; feeding device (1) communicated with main body and placed on middle section of gasification furnace main body; an upper steam nozzle (5) placed in clearance area; lower carbon dioxide/steam nozzle (4) placed on bed area; synthetic gas outlet on the top part of the gasification furnace main body and monitoring unit arranged on the outlet; and a microwave plasma generator (3) placed in clearance area and placed at the upper end of upper steam nozzle.    USE - As an external heating-type microwave plasma gasification furnace for producing synthetic gas (claimed) i.e. biomass fuel.    ADVANTAGE - The external heating-type microwave plasma gasification furnace is efficient and economical; provides synthetic gas with the content of more than 90%; is simple for manufacturing; and has good economical efficiency.    DETAILED DESCRIPTION - An external heating-type microwave plasma gasification furnace, comprises a gasification furnace main body (2) vertically arranged, a feeding device (1) communicated with the main body and placed on the middle section of the gasification furnace main body, an upper steam nozzle (5) placed in a clearance area, a lower carbon dioxide/steam nozzle (4) placed in a bed area, a synthetic gas outlet on the top part of the gasification furnace main body and a monitoring unit arranged on the outlet, and a microwave plasma generator (3) placed in the clearance area and placed at the upper end of the upper steam nozzle; 1-2 layers of microwave plasma generators are arranged, each layer is uniformly distributed with 3-4 plasma work gas access points, the plasma current is radially or tangentially sprayed into the clearance area of the gasification furnace main body, where the gasification furnace further comprises an external heating device for heating the gasification furnace by an external heat source, where the external heating device is arranged integrally with the gasification furnace main body or is arranged by separating from the gasification furnace main body, and where the external heating device is provided with a slag hole for discharging the ash. An INDEPENDENT CLAIM is included for producing synthetic gas by the gasification furnace, which takes an external energy source as a heat source of the biomass gasification technique for gasifying the biomass fuel, involving (a) feeding the biomass fuel and garbage into the gasification furnace main body by the feeding device, rapidly gasifying the materials on a high-temperature bed for performing complex and efficient pyrolytic reaction for mainly generating high-temperature synthetic gas comprising carbon monoxide, hydrogen, carbon dioxide and a few amount of methane, tar etc.; (b) moving the high temperature synthetic gas upwards until reaching the clearance area of the gasification furnace, rapidly performing cracking reaction under the action of a catalyst which is rich in active high temperature plasma which is high in degree of ionization and is in a non-equilibrium state and is generated by the microwave plasma generator, at the same time, spraying into steam by the upper steam nozzle, controlling the temperature of the clearance reaction area at 1000-1200 degrees C; controlling the speed of the plasma gas for ensuring that the retention time of the synthetic gas in the plasma range of the clearance area is 3-10 seconds, increasing turbulent flow for intensifying the heat transfer and the mass transfer in the flow field; introducing the generated synthetic gas finished product to a following technique by a synthetic gas outlet of the gasification furnace; (c) enabling the remained part fixed carbon on the bed to exist in the bed material, spraying into high-temperature CO2/high temperature steam gas by the lower nozzle for performing redox reaction for consuming part of the fixed carbon; (d) transporting the remained residual carbon which is not completely reacted, and the residue and the bed material into the external heating device by the circulating bed material outlet for combusting the residual carbon, heating the bed material and separating the bed material from the residue in the external heating device, discharging the separated ash by the slag hole arranged on the external heating device; (e) transporting the separated bed material to the circulating bed material inlet for introducing into the gasification furnace for exchanging heat with the high-temperature smoke by countercurrent flow, continuously moving the bed material downwards, discharging the heat when the bed material contacts with the fuel substance on the bed, reducing the temperature until the temperature of the bed reaction area reaches 600-1000 degrees C; introducing the cooled bed material into the heating device once gain, circulating the operation for multiple times, where the temperature of the circulating bed material is 750-1200 degrees C, the temperature of the heated high-temperature bed material is higher than that of the bed reaction area; and (f) when performing the above steps, simultaneously, monitoring the temperature and the component of the synthetic gas by the monitoring unit on the synthetic gas outlet, regulating the power of the microwave plasma and the supply quantity of the carbon dioxide and the steam in real time for ensuring the technique to be stably and economically operated.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of ion gasification furnace.    Feeding device (1)    Gasification furnace main body (2)    Microwave plasma generator (3)    lower carbon dioxide/steam nozzle (4)    Upper steam nozzle (5)