Use of polysaccharides from Radix isatidis in the manufacture of medicaments for treating and/or preventing diseases induced by influenza virus and complication thereof, wherein the molecular weight of the polysaccharides from Radix isatidis is 3000-7000Da, and the influenza virus includes influenza virus A, influenza virus B, bird influenza virus, such as human influenza virus H1N1, H3N2, bird influenza virus H6N2, H7N3, H9N2 and INF B. The mechanism of the polysaccharides from Radix isatidis against influenza virus is due to the ability to inhibit the binding of influenza virus to the host cell. The present invention also discloses a pharmaceutical composition containing the polysaccharides from Radix isatidis and the method for preparing the polysaccharides.