Abstract: It is an object of the present invention to provide a method for preparing a water extract of ashwagandha, which comprises increasing the amounts of withanone and withaferin A contained as active ingredients in the water extract of ashwagandha leaves. In addition, it is another object of the present invention to more economically and simply provide a pharmaceutical composition comprising ashwagandha leaves. The present invention relates to a method for preparing a water extract of ashwagandha leaves, which comprises extracting ashwagandha leaves with water in the presence of cyclodextrin, and a method for enhancing the anticancer activity of the water extract of ashwagandha leaves. The present invention also relates to a pharmaceutical composition for treating or preventing cancer, comprising the water extract of ashwagandha leaves. The present invention further relates to a pharmaceutical composition comprising dry powders of ashwagandha leaves and cyclodextrin.