The invention relates to the field of a feed additive, particularly relates to a selenium-rich feed additive and a preparation method of the feed additive. The selenium-rich feed additive is cardamine hupingshanensis dry powder with the selenium concentration being 1700-2500ppm. The preparation method of the selenium-rich feed additive comprises the following steps of: transplanting and cultivating cardamine hupingshanensis plant with turfy soil as matrix; carrying out root-irrigation with sodium selenite with the concentration being 250-500ppm after transplanting and cultivating for two months; 20 days before bolting, mixing sodium selenite with the concentration being 300-700ppm and gibberellins with the concentration being 50-90ppm and spraying the mixture to cardamine hupingshanensis plant; and drying and sterilizing the pure cardamine hupingshanensis plant at 80-110 DEG C, then crushing to obtain selenium-rich cardamine hupingshanensis dry powder with the selenium concentration being 1700-2500ppm. The selenium-rich feed additive can enhance the ability of livestock and poultry, thereby preventing disease, improving immune ability and oxidation resistance of livestock and poultry, improving the egg yield and milk yield of animals, and prolonging the period of egg laying and milk producing products; moreover, the feed additive is safe and non-toxic.