The invention relates to a preparation method of synthesis gas for synthesizing acetic acid, mainly aiming at solving the problem that raw gas obtained through pressured gasification of coal-water slurry in the prior art cannot meet the acetic acid production demand. The preparation method comprises the following steps of: 1) taking water gas as raw materials, and dividing the raw materials 1 into a material flow 2, a material flow 3 and a material flow 4, wherein the volume of the material flow 2 is 10-50 percent of the volume of the raw materials 1, the volume of the material flow 3 is 10-40 percent of the volume of the raw materials 1 and the volume of the material flow 4 is 35-60 percent of the volume of the raw materials 1; b) feeding the material flow 2 into a CO conversion reactor to obtain a material flow 5 containing CO2 and H2; c) feeding the material flow 4 into a gas separation device to obtain a material flow 6 and a material flow 8 after separation; d) feeding the material flow 3 and the material flow 5 into an acid gas washing device, then mixing the washed material flow with the material flow 6 to obtain a material flow 7; and e) feeding the material flow 7 as synthesis gas for synthesizing methanol into a methanol synthesizing unit, and feeding the material flow 8 into an acetic acid synthesizing unit, wherein the material flow 7 and the material flow 8 are the synthesis gas for synthesizing the acetic acid. By adopting the technical scheme, the problem is better solved and the preparation method can be used for the industrial production of the synthesis gas for synthesizing the acetic acid.