NOVELTY - The method comprises performing low temperature biomass pyrolysis reaction by spiral feeder (2) to pre-treated biomass particles into a pyrolysis furnace to a reaction furnace for low temperature thermolysis reaction at normal temperature (300-600 degrees C) and under normal pressure to prepare coke/semi-coke and pyrolysis gas, performing semi-coke/coke high temperature gasification at 700-900 degrees C by a low temperature pyrolysis gasification furnace (6) down to the reaction furnace by the carrier gas, and releasing coke tar/raw synthesis gas catalytic reforming little tar and the crude synthetic gas.    USE - The method is useful for preparing synthesis gas via three-section biomass pyrolysis-gasification-catalytic reforming, where the synthesis gas is useful for synthesizing diesel oil/gasoline, dimethyl ether, low carbon alcohol, synthesis and preparing liquid fuel.    ADVANTAGE - The method is capable of simply, efficiently and economically preparing synthesis gas with high quality and carbon conversion rate and without generation of secondary pollution.    DETAILED DESCRIPTION - The method comprises performing low temperature biomass pyrolysis reaction by spiral feeder (2) to pre-treated biomass particles into a pyrolysis furnace to a reaction furnace for low temperature thermolysis reaction at normal temperature (300-600 degrees C) and under normal pressure to prepare coke/semi-coke and pyrolysis gas, performing semi-coke/coke high temperature gasification at 700-900 degrees C by a low temperature pyrolysis gasification furnace (6) down to the reaction furnace by the carrier gas, and releasing coke tar/raw synthesis gas catalytic reforming little tar and the crude synthetic gas to the reaction furnace catalyst bed layer (5) inside the mixed gas flow of vapor or steam/air under the effect of the catalyst of the catalytic reforming reaction to prepare refined synthesis gas at 750-850 degrees C. The three reaction steps are performed using a pyrolysis furnace on the upper section in the gasification reaction device. A lower segment gasification furnace and a catalyst bed layer have continuous space. A biomass raw material in the pyrolysis reaction through low temperature pyrolyzing coke or semi-coke and pyrolysis gas in fluidized bed gasification reaction chamber in the reaction for preparing the rough synthesis gas by coke or high temperature vapor gasification semi-coke on the catalyst bed layer is formed by performing catalytic cracking for pyrolyzing tar and catalytic reforming pyrolysis gas for reducing the content of tar yield to obtain the high quality synthesis gas. The mixed gas of vapor or steam/air in the bottom of the fluidized bed are the fully fluidized and at the same time carrying out coke steam or high temperature semi-coke gasification reaction to produce raw synthesis gas. The pre-process of the biomass particles is less than 20%. The biomass particle has a granularity of 0.5-5 mm. An upper part (4) of the pyrolysis furnace, the lower segment gasification furnace and the middle catalyst bed layer are present in the same reaction furnace body. The upper part of pyrolyzing furnace is normal pressure gas-solid counter current moving bed. The lower segment gasification furnace is arranged for bubbling fluidized bed. The catalyst bed is a fixed bed. The carrier gas air flow speed of the raw material is controlled 5-30 s the staying time of the pyrolyzing furnace. The raw material is controlled 2-10s in the residence time of the gasification furnace. The fluidized bed material filling quantity and unit time biomass ratio of feeding amount kg is 0.25-0.5:1. The tar catalytic cracking the mixed gas of vapor or steam/air is performed in a nickel-based catalyst bed layer of the raw synthesis gas and the catalytic reforming reaction. The nickel-based catalyst consists of industrial load type M basic catalyst, olivine load between group catalyst, perovskite structure M group, or the nickel base catalyst, catalyst dosage is 2-10% of the weight of the biomass raw material.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a device for production of synthesis gas (Drawing includes non-English language text).    Biomass storage bin (1)    Spiral feeding machine (2)    Upper part of pyrolyzing furnace (4)    Catalyst bed layer (5)    Lower segment gasification furnace. (6)