The invention discloses a method for preparing citric acid by fermentation. The method comprises the steps of grafting a citric acid fermentation strain into a fermentation culture medium and fermenting to generate citric acid. The fermentation process comprises a strain growth stage and a strain acid-production stage, wherein the fermentation temperature in the strain growth stage is 30-38 DEG C; the fermentation temperature in the strain acid-production stage is 32-42 DEG C; and the fermentation temperature in the strain growth stage is lower than that in the strain acid-production stage. By virtue of adopting the technical scheme for producing citric acid, the citric acid concentration and the fermentation conversion rate are effectively increased, the fermentation cycle is shortened, and the energy consumption is lowered.