The invention relates to a feed additive for preventing feed from being subjected to fat oxidation and mildew as well as a preparation method and application thereof and belongs to the field of feeds.The feed additive comprises the following components in parts by weight: 2-10 parts of sage, 3-15 parts of a sophora Japonica extract, 0.2-1 part of resveratrol, 0.2-1 part of madder pigment, 1-5 parts of an acidifying agent and 68-93.6 parts of a carrier. The acidifying agent comprises at least one of fumaric acid and tartaric acid. The feed additive is safe, non-toxic, small in dose, low in cost and stable in effects. The preparation method comprises the step of mixing the components of the feed additive, so as to obtain the product. The preparation method is simple, and the condition controllability is excellent. When the feed additive is applied to preparing pig feed, the oxidation and mildew degree of the feed can be reduced or relieved, and when the pig feed is used for feeding pigs, the growth performance of the pigs can be improved, and the feed conversion ratio is reduced.