The invention relates to a method for preparing casein. The method comprises the following steps of: preparing reconstituted milk by taking substandard milk powder, crude casein, liquid milk, and the like as main raw materials, precipitating the casein by utilizing the characteristic that the isoelectric pointof the casein is 4.6 and utilizing dilute hydrochloric acid as a coagulator to be directly used for the precipitation process of the casein, breaking the structure of lipid-protein compound in the milk powder by utilizing an ammonia solution, dissociating combined fattiness, extracting the fattiness by utilizing ethyl ether and petroleum ether, separating an organic layer, agglomerating the casein again by utilizing the dilute hydrochloric acid again, washing and dehydrating the casein to obtain half-wet casein blocks, and drying granulated casein in a two-section mode to obtain finished casein products. The invention greatly enlarges the types of raw materials for preparing the casein, can utilize the substandard milk powder, the crude casein, fresh milk with excessive acidity and fresh milk containing antibiotics to be not suitable for processing and utilization, and has the advantages of simple production process, low cost, high product purity, complete utilization of resources, recycled solved, safe production process and easy control of quality.