The present invention provides a pharmaceutical composition for treating rotavirus and/or respiratory syncytial virus infection and a method for preparing the same; wherein said pharmaceutical composition comprises a therapeutically effective amount of Spirulina extract and a pharmaceutically acceptable carrier. The Spirulina extract used in the present invention is extracted at a low temperature by the following steps: (a) adding an organic Spirulina powder into a hypotonic buffer solution and mixed well to obtain a mixture; (b) freezing the mixture from step (a) overnight at a temperature lower than 0.degree. C.; (c) thawing the mixture from step (b); (d) separating and purifying the mixture from step (c) by a separator, and collecting blue fractions; and (e) spray drying the blue fractions from step (d).