The invention relates to a Brasenia schreberi polysaccharide preparation method. The preparation method comprises the following steps: drying a raw material fresh Brasenia schreberi, and crushing to obtain Brasenia schreberi powder; adding water to the Brasenia schreberi powder for dilution, adjusting the pH value of the obtained solution, adding papain, carrying out enzymatic hydrolysis treatment under controlled conditions, filtering, concentrating, adding a mixture containing organic solvents comprising chloroform and n-butanol, and stirring for removing proteins; carrying out dialysis desalting through a dialysis membrane, carrying out alcohol precipitation, collecting the obtained precipitate, washing, and drying to obtain crude Brasenia schreberi polysaccharides. The preparation method has a high protein removal rate, a small polysaccharide loss and a good separation and purification effect, and determination results show that the protein removal rate reaches above 80%, and the purity of the obtained crude Brasenia schreberi polysaccharides reaches above 70%. The preparation method has the advantages of reasonable technology, advanced technique, feasible operation, and suitableness for the large-scale factory production.