The invention relates to a premixed feed additive for alleviating immunosuppression of weaned piglets and a preparation method thereof. The premixed feed additive comprises the following components by weight: 240-400 parts of a polysaccharide compound, 160-250 parts of a yeast extract, 140-220 parts of vitamin E, 20-32 parts of vitamin B12, 180-240 parts of soybean isoflavone, and 25-50 parts of tea polyphenol. The polysaccharide compound is prepared from the following raw materials by weight: 1-4 parts of Astragalus, 2-4 parts of bighead atractylodes rhizome, 1-3 parts of Chinese yam spray-dried powder, 0.5-2 parts of Caesalpinia sappan, and 0.2-1 part of alga. During preparation, the polysaccharide compound, the yeast extract, vitamin E, vitamin B12, soybean isoflavone and tea polyphenol can be mixed evenly directly. Compared with the prior art, the additive provided by the invention has strongly effective effect in alleviating immunosuppression. At the same time, the premixed feed additive also has the effect of obviously improving the immunity of non-immunosuppressed piglets, and can effectively improve the immune effect of vaccines.