The present disclosure discloses preparation of M. alpina CCFM698 thalli and application thereof in a feed additive, and belongs to the field of biological engineering and feed additives. The total fatty acid content of the M. alpina dried thalli obtained by the present disclosure is 30%-40% by weight of the dried thalli, and the EPA content is 24% or more by weight of total fatty acids. The dosage of the dried thalli in the feed in the disclosure is 0.5-1.5% of the total weight of the basal feed. The thallus feed additive is reasonable in fatty acid composition and very high in EPA content and can be used for producing high-DHA eggs which are beneficial to body health of eaters. The DHA content in each of eggs laid by laying hens fed with the feed containing the feed additive of the present disclosure reaches about 120 mg which is obviously higher than that in the prior art.