After the corn is crushed by the dry milling way, the corn slurry enters the ejection & liquefaction system. Maintain the temp., and the liquefaction for a certain time, it enters the filtration section. Filter the crude protein, after sterilization, it enters the fermentation system. Maintain the certain temp., certain time and sufficient oxygen for the fermentation. After filtering, the fermentation mash enters the neutralization section of the citric acid production. After washing and recovering the citric acid, the filtered bacterial residue is mixed with the liquefied crude protein for drying as the animal feed. In the neutralization section, adopt two times neutralization and filtration, calcium hydrogen citrate precipitation is obtained . After acid hydrolysis and extraction, primary citric acid is obtained. Through decolorization, filtration and continuous ion exchange system, the colored impurities, peculiar smell and other impurity ions of syrup are removed, so as to make the citric acid achieve the purpose of refining and purification. After that, it goes into the vacuum evaporation crystallization system, after centrifugal separation, drying, the qualified citric acid anhydrous is obtained. The mother liquor coming from centrifuge goes through the vacuum evaporation & crystallization system, centrifugal separation and drying, to get the qualified citric acid monohydrate as the final product.