An inhibitor for resisting muskmelon powdery mildew as well as preparation method and application thereof are provided, belonging to the technical field of muskmelon culturing. In order to solve the technical problems that muskmelon powdery mildew has caused serious influence on growth and yield of muskmelon, the disclosure uses an inhibitor containing allantoin for resisting muskmelon powdery mildew to spray leaves of the muskmelon in a seedling stage or a spreading stage of the muskmelon, which effectively increases the resistance of the muskmelon to powdery mildew. The disclosure has great significance for improving the yield and quality of the muskmelon.