The invention provides an application of polysaccharide sulfate in preparation of a medicine for preventing or treating tumor metastasis. The polysaccharide sulfate has the advantages of being high insafety and small in bleeding side effect, the polysaccharide sulfate is derived from the ocean, raw materials are easy to obtain, and the price is low. The experiments further prove that the polysaccharide sulfate has the effect of inhibiting tumor lung metastasis and liver metastasis, the effect is remarkable, the clinical indications of the polysaccharide sulfate are increased, the defects of the anti-tumor metastasis drug market are overcome, and the polysaccharide sulfate has important economic and social values.