A method for preparing a liquid hydrocarbon from syngas. The method includes: 1) mixing crude syngas from a biomass gasifier and a hydrogen-rich gas to yield a mixed gas; 2) dehydrating and decarbonizing the mixed gas for removal of moisture, carbon dioxide, and impurities, to yield a fine syngas; 3) introducing the fine syngas to a Fischer-Tropsch synthesis device in the presence of a catalyst, controlling a reaction temperature of the Fischer-Tropsch synthesis at between 150 and 300° C. and a reaction pressure of between 2 and 4 MPa (A), to yield a liquid hydrocarbon and water which is discharged out of the Fischer-Tropsch synthesis device; and 4) returning 70-95 vol. % of exhaust gases from the Fischer-Tropsch synthesis device to step 3) to mix with the fine syngas, and introducing the resulting mixed gas to the Fischer-Tropsch synthesis device.