The invention discloses a method for using membrane separation and classification to prepare soybean polysaccharides with different molecular weights. The method includes the steps of after cooking at high temperature under an acid environment to obtain separated solid and liquid of the soybean polysaccharides, firstly adopting an inorganic ceramic membrane system for separation to obtain clean liquid of soybean polysaccharides, then using an ultrafiltration membrane with 400, 000 molecular weight for ultrafiltration and subsequently an ultrafiltration membrane with 200, 000 molecular weight for ultrafiltration, and finally collecting ultrafiltrate respectively with the molecular weight more than 400, 000, between 400, 000 and 200, 000 and less than 200, 000 to obtain three soybean polysaccharides solutions with different molecular weight and then soybean polysaccharides powder with different purposes after concentrating and freezing or spray-drying the soybean polysaccharides solution. By obtaining the polysaccharide through precipitation without adding organic solvent alcohol, the method has the advantages of saving energy, causing no pollution, saving time and the like, can be applied to amplified production in industrial practice, and the soybean polysaccharides with various molecular weights after membrane classification purification are applicable to different food industries, thus widening application range with obvious effect.