A method for preparing purified extract containing more abundant active ingredients such as catalpol derivatives from the extract of Pseudolysimachion rotundum var subintegrum than that prepared by the conventional preparation method disclosed in the prior art and the therapeutics or functional health food comprising the purified extract for treating and preventing inflammatory, allergic or asthmatic disease. The purified extract showed more potent anti-inflammatory, anti-allergy and anti-asthma activity than that prepared by the conventional preparation method disclosed in the prior art through various in vivo tests such as inhibition test on the reproduction of eosinophil, the release of immunoglobulin and inflammatory chemokines in plasma and bronchoalveolar fluid as well as the suppression of airway hyperresponsiveness and goblet cell hyperplasia in a OVA-sensitized/challenged mouse model.