The invention relates to a preparation method of renninum, belonging to the technical fields of bioengineering and preparation of enzyme preparations. The preparation method is mainly characterized by comprising the following steps of: (1) preparing shake flask seeds; (2) inoculating: inoculating a liquid strain onto a bran culture medium; (3) performing fermentation culturing at 33 DEG C for 48 hours; (4) leaching: leaching the material with a 9 percent salt solution overnight to obtain a crude enzyme solution; (5) centrifuging: centrifuging the crude enzyme solution at the speed of 8,000-10,000 revolutions per minute; (6) ultra-filtering: removing foreign proteins with an ultra-filtering film of 8k; and (7) performing freeze drying: adding a cryoprotectant, and performing freeze drying in vacuum to obtain finished renninum. In the invention, bran is taken as a major raw material, so that the preparation method has the advantages of low raw material price, simple operating process, short fermenting period, saving in time, economic efficiency, reduction in the production cost and suitability for industrial production and popularization.