The invention discloses a method for using a multi-element mixed gasification agent to prepare a synthesis gas by dry slagging of a fixed bed, which comprises the steps of: preparing the multi-element mixed gasification agent consisting of steam, CO2 and oxygen in a ratio that steam: CO2: oxygen is 0-8kg:1-7Nm3:1Nm3, wherein the addition amount of steam is decreased with the increase of the addition amount of CO2; enabling the multi-element mixed gasification agent to enter a dry slagging gasification furnace of the fixed bed from the gas inlet of the gasification furnace via a gas distribution device (grate), wherein the addition amount of the multi-element mixed gasification agent is in a ratio that coal:oxygen is 5-8kg:1Nm3; and gasifying under the condition that the temperature of the inlet of the gasification agent is 250-400 DEG C, the gasification pressure is 2.5-6MPa and the gasification temperature is 1000-1400 DEG C. The invention has the advantages of reducing steam consumption and CO2 emission.