A method for producing synthesis gas, including: 1) pre-treating a biomass raw material; 2) carrying out low-temperature carbonization to obtain pyrolysis gas and charcoal, cooling the charcoal at an outlet of an carbonization furnace, and conveying the cooled charcoal to a charcoal storage bin; 3) separating the pyrolysis gas from charcoal powder; 4) delivering part of a separated pyrolysis gas to a combustion bed for combustion, heating the other part of the separated pyrolysis gas, and delivering a heated pyrolysis gas to the carbonization furnace; delivering a waste hot flue gas after heat exchange to a pretreatment part for the biomass raw material for drying; conveying the separated charcoal powder to the charcoal storage bin; 5) milling the charcoal powder to prepare a charcoal slurry; and 6) using high-pressure charcoal slurry pump, introducing the charcoal slurry to a gasification furnace for gasification.