The invention belongs to the field of deep processing of aquatic products, in particular relates to a mollusk mucus polysaccharide and a preparation method thereof, the polysaccharide powder is prepared by taking mucus of living yellow mud snails as raw material, the yield of the crude polysaccharide is 5 percent-9 percent, the yield of the polysaccharide after the purification is 1 percent-3 percent, and the weight of the yellow mud snail mucus polysaccharide which is measured by the phenol-sulfuric acid method is 82 percent-90 percent. The production process comprises extraction, concentration, sedimentation, purification, separation, drying and other steps. The mucus polysaccharide of the provided product has higher effect of inhibiting tumor cell proliferation and black pigment formation. The provided production process is simple and has universal adaptability for the mucus extraction of living mollusk; furthermore, the extraction rate of the polysaccharide is high and the purity is high.