A method for purifying biomass syngas under a positive pressure. The method includes: 1) aerating a pyrolysis gasifier by using an oxidation fan; 2) introducing syngas produced in the pyrolysis gasifier through a water-cooling flue to a water-cooling quench tower; 3) introducing the syngas from the water-cooling quench tower to a waste heat boiler of a water-tube type and a waste heat boiler of a heat-tube type; 4) introducing the syngas from the waste heat boiler of the heat-tube type to a Venturi scrubber in the absence of a filler to wash the syngas and remove dust; 5) introducing the syngas from the Venturi scrubber to a wet electrical dust precipitator for further removing the dust; and 6) transporting qualified syngas to a wet gas tank for storage or to a downstream process for use.