The present invention relates to: a pharmaceutical preparation which contains entecavir as an active ingredient and to which an antioxidant is added in order to improve the stability of entecavir against moisture; and a preparation method therefor. According to the pharmaceutical preparation and the method, entecavir can be stably maintained in the pharmaceutical preparation, which contains a predetermined content or more of water, by adding the antioxidant, and therefore the entecavir can be provided as a pharmaceutical preparation in a dosage form containing a predetermined content or more of water, such as a dosage form of an orally disintegrating film.