A process of using Isatis root polysaccharide in the manufacture of medicaments for treating and/or preventing diseases induced by influenza virus and complications thereof, wherein the molecular weight of the Isatis root polysaccharide is 3000-7000 Da, and the influenza viruses include influenza virus A, influenza virus B, avian influenza virus, such as human influenza virus H1N1, H3N2, avian influenza virus H6N2, H7N3, H9N2 and INF B. The mechanism of the Isatis root polysaccharide against influenza virus is due to the ability to inhibit the attachment of influenza virus to the host cell. The present invention also discloses a pharmaceutical composition containing the above-mentioned Isatis root polysaccharides and a method for preparing the polysaccharides.
