A method for blowing the annular gap of a gasification furnace in a mixed gasification technology is characterized in that a gasification furnace annular gap blowing gas is changed to CO2 or purified synthetic gas or a combined gas from N2 in original technologies, and CO2 and the purified synthetic gas are added based on the original N2 used as the gasification furnace annular gap blowing gas in order to form a present annular gap blowing gas; CO2 is used in the startup and shutdown process of the gasification furnace and the break-down working condition of purified synthetic gas supply, the purified synthetic gas is used in the normal running working condition of the gasification furnace, and N2 is used as an emergency gas source, and is used as an emergency blowing gas in the gas source supply goes wrong. The method has the advantages of realization of no differential pressure of the water wall of the gasification furnace, prevention of corrosion of annular gap internal pieces on the gasification furnace, reduction of the energy consumption of a circulating gas compressor in downstream methanol synthesis, reduction of the vent gas amount, and guaranteeing of long-term stable running of the gasification furnace.