The invention discloses a preparation method of a citric acid fermentation solution, which comprises the steps of: (1) mixing dry corn powder with water to form paste, adding amylase into the paste and then heating the paste, conducting spraying liquefaction by one time on the paste at the temperature ranging from 92 DEG C to 98 DEG C and then flash evaporation by one time, lowering the temperature to 88 DEG C to 90 DEG C and maintaining the liquefaction for 90min to 120min to obtain liquefied solution; (2) taking 5 percent to 15 percent of the liquefied solution obtained in the step (1), putting the liquefied solution into a seed tank, adding a nitrogen source, heating and sterilizing, maintaining for 20min to 30min, then lowering the temperature to 36 DEG C, and inoculating the obtainedmixture into aspergillus niger strains for strain culture; (3) taking 5 percent to 15 percent of the liquefied solution obtained in the step (1), putting the liquefied solution into a fermenter; (4) taking the rest of the liquefied solution obtained in the step (1) for solid-liquid separation to obtain liquefaction clear liquid, putting the obtained liquefaction clear liquid into the fermenter, adding a nitrogen source, raising the temperature to 80 DEG C to 90 DEG C by heating, sterilizing, maintaining for 5min to 10min, lower the temperature to 38 DEG C, and leading the substances in the seed tank obtained in the step (2) into the fermenter; and (5) conducting solid-liquid separation to obtain the citric acid fermentation solution after ferment. The preparation method has the advantagesof simple technique and low production cost.