Disclosed herein are brown algae-derived fucoidan and in particular a method for degrading fucoidan to a low-molecular weight so that fucoidan as sparingly-degradable polysaccharide (M.W. 3,000,000 to 5,000,000) can be more efficiently extracted from brown algae and it can be absorbed in the human body. The method comprises preparing a fucoidan extract containing as much fucoidan as possible from brown algae selected from Hizikia fusiforme, Undaria pinnatifida, Laminaria sacchalina, Ecklonia cava, Cladosiphon Okamuranus and Nemacystus decipiens, and low-molecularizating the fucoidan extract using Vibrio halioticoli (Accession No. KCTC 10998BP) and a probiotic mixture containing Streptococcus faecalis, Bacillus mesentericus, Clostridium butyricum, and glycosyltransferase thereof. According to the method, fucoidan can be efficiently degraded to a low molecular weight using microorganisms so that it can be absorbed in the human body. The fucoidan obtained realizes practical utilization of beneficial efficiencies thereof in a variety of fields, in particular, health foods, meals for patients (e.g., cancer patients), medicine, cosmetics, agriculture, fishing and stockbreeding.