The invention discloses a preparation method of low-molecular-weight sea cucumber fucosan sulfate. In the preparation method, a sea cucumber polysaccharide prepared by performing enzymolysis on pulverized sea cucumbers is used as a raw material. The preparation method comprises the following steps: 1) dissolving the sea cucumber polysaccharide in a sodium chloride solution, then adding ethanol, and centrifuging to remove the precipitate, thus obtaining the supernatant; and adding ethanol into the supernatant, centrifuging, dialyzing the obtained precipitate, and performing freeze-drying to obtain sea cucumber fucosan sulfate; 2) dissolving the sea cucumber fucosan sulfate in solvent, and performing ultrasonic treatment at 0-45 DEG C, wherein the treatment time is 10-180 minutes, and the ultrasonic sound field strength is 60-800 W/cm<2>; and 3) dialyzing the treated solution obtained in the step 2), and performing vacuum freeze-drying to obtain the low-molecular-weight sea cucumber fucosan sulfate. According to the invention, an ultrasonic degradation technology is adopted, high-molecular-weight sea cucumber fucosan sulfate can be quickly degraded, the reaction conditions are mild, and the preparation method is green and environment-friendly.