The invention relates to the technical field of oyster cultivation, in particular to selenium-rich oyster feed and a preparation method thereof.The selenium-rich oyster feed is prepared from, by mass, 18-32 parts of selenium-rich corn flour, 8-14 parts of sweet potato powder, 9-15 parts of selenium-rich spirulina, 7-9 parts of coscinodiscus, 7-9 parts of thalassemia, 3-5 parts of shrimp shell meal, 3-5 parts of fish bone meal and 2-6 parts of silkworm chrysalis meal. 10-18 parts of sesame oil residues, 5-8 parts of a traditional Chinese medicine mixed solution, 2-3 parts of a compound vitamin additive, 1-1.5 parts of selenium-enriched yeast and 0.2-0.4 part of cellulase. According to the invention, the selected coscinodiscus and thalasiosira in the raw materials are natural biological baits for oysters in a natural sea area environment, and the added selenium-enriched spirulina enables the content of selenium in the feed to be rich, so that the feed also has relatively strong oxidation resistance and immune function, the immunity of the oysters is improved, and the feed is suitable for being eaten by the oysters. The feed can effectively enhance the physique of the oysters, reduces the prevalence rate of the oysters, also reduces the breeding labor amount and the breeding cost, and is safer to eat aquatic products.