The invention relates to semi-artificial diet of ophraella communa Lesage, the natural enemy of exotic ragweed, and a preparation method thereof, which belong to the field of biological control, and the semi-artificial diet of ophraella communa Lesage subsequently comprises the following 8 key components: wheat germ, 10 percent of KOH, glucose, nipagin, ragweed leaf powder, agar, cholesterol and soybean oil; the 8 key components which affect the growth of ophraella communa Lesage larvae are as follows: oatmeal, agar, wheat germ, glucose, choline chloride, 10 percent of KOH, distilled water and soybean oil. In the preparation process of the semi-artificial diet, the preparation of the ragweed leaf powder, the sequence of all the components in preparation, the preparation method and the sterilization and storage methods are standardized, so as to feed and reproduce ophraella communa Lesage, the natural enemy of exotic ragweed in a large scale.