Abstract: In order to evaluate the breeding effect of the culture of Paecilomyces militaris FY1421 as a feed additive for Penaeus vannamei, The mycelium with high cordycepin content and its metabolites, the culture of Paecilomyces militaris FY1421, were obtained by liquid fermentation, which was added as an additive to the basal diet of shrimp at 0.05%, and at the same time, the basal diet was set as the control group, thus to investigate the effects of adding the culture of Paecilomyces militaris FY1421 on the growth of Penaeus vannamei. The results showed that the weight gain rate and survival rate of Penaeus vannamei in the experimental group were 34.0% and 24.8% higher than those in the control group, respectively, and the difference reached a significant level (P<0.05). At the same time, the feed coefficient of the experimental group decreased by 9.8%. Compared with the control group, the experimental group could increase the activity of serum lysozyme and phenoloxidase of shrimp by 43.3% and 34.0%, respectively, and the difference reached a significant level (P<0.05). The alkaline phosphatase, superoxide dismutase and peroxidase in hepatopancreas also increased significantly (P<0.05), and the content of malondialdehyde in muscle decreased by 37.9%. The results showed that the culture of Paecilomyces militaris FY1421 could improve the growth performance of Penaeus vannamei, improve the immune activity and antioxidant capacity of shrimp, and could be beneficial to the survival and disease resistance of shrimp.