The invention provides a preparation method of aspergillus niger bran koji. The reparation method comprises the following steps: first washing bran koji preparing raw material bran or a corn cob, and centrifugally dewatering the washed bran or corn cob to reduce the water content of the raw material to 55-70%, then putting the bran or corn cob into a special culturing steel bottle for the aspergillus niger bran koji, sterilizing, inoculating the steel bottle with aspergillus niger, putting the steel bottle onto a bottle rocker, culturing, directly inoculating a seed tank with the qualified aspergillus niger bran koji, and performing amplification culture. The automatic/semiautomatic mechanical equipment and the matched culturing steel bottle are used for producing the aspergillus niger bran koji, an inherent mode that in the conventional bran koji preparing process, a large number of triangular bottles are used, the production process is large in labor quantity and cumbersome and standardized production cannot be achieved is changed. When the preparation method is used for culturing the aspergillus niger bran koji, large-scale industrial production is facilitated, the labor intensity is reduced, the production efficiency is improved, and the labor cost is saved in the case of balanced acid production, so that economic benefits are good.