The depolymerization effect of UV/HO on the polysaccharides from Sargassum fusiforme (PSF), a brown algae, were studied. The structural changes of PSF before and after UV/HO treatment were analyzed, and molecular weight changes during in vitro digestion were determined. Results indicated that the molecular weight of PSF was reduced from ∼289 to ∼12.6 kDa within 2 h with UV/150 mmol/L HO, and the depolymerization effect of UV/HO was significantly higher than that of UV or HO alone. In addition, the UV/HO treatment had a high recovery rate of total sugar (93.54 %) and clearance rate of protein (76.34 %). The monosaccharide composition showed that UV/HO treatment could increase the mole percentage of mannose (37.44 %) and decrease the mole percentage of fucose (14.88 %). The helix-coil transition, X-ray diffraction (XRD) and atomic force microscopy (AFM) imaging showed that the UV/HO treatment depolymerized PSF. Rheological studies indicated that PSF with UV/HO treatment had lower viscosity. In vitro digestion showed that PSF was minimally digested with the in vitro gastrointestinal tract simulation, but PSF with UV/HO treatment could be digested in the low acid environment in the simulated gastric juice, but was minimally digested with the simulated intestinal juice. This studied suggested that the preparation and application of functional PSF with low molecular weight might be beneficial.