Objective To detect the antiviral effect and its antiviral mechanism of the sulfated polysaccharide SP2 isolated from the brown alga Sargassum patens C. Ag. against RSV.Methods The antiviral activity,cytotoxicity and its possible antiviral mechanism were assayed by cytopathogenic effect (CPE) inhibition assay,MTT assay and plaque reduction assay. Reduction assay when it was added during virus adsorption. It did not show any antiviral activity against influenza A and B viruses and parainfluenza type 3 virus up to the concentration of treatment of cells with SP2 did not protect the cells from RSV infection. This polysaccharide addition assay showed that when SP2 was present only during virus adsorption phase at 37℃, it could significantly inhibit RSV-induced plaque formation. Conclusions SP2 might be a potent substance for treating RSV infection. The mode of action of SP2 might depend on inhibiting RSV attachment to host cells.