Studies were conducted to evaluate polysaccharides from mycelia of Cordyceps sinensis (PMCS) as a feed additive for pacific white shrimp (Litopenaeus vannamei). Shrimps were randomly divided into two groups. One group received the PMCS-supplemented diet for 52 days, and the other group received no treatment. The data showed that shrimps given PMCS gained more body weight compared with the control group (P < 0.05). Haemolymph immunity indicators, including phenoloxidase (PO), alkaline phosphatase (ALP), acid phosphatase (ACP) and lysozyme (LSZ), were remarkably greater in the PMCS group (P < 0.05). In addition, antioxidant parameters, including superoxide dismutase (SOD), glutathione reduced form (GSH), reactive oxygen species (ROS) and total antioxidant capacity (TAC), were significantly greater in the treatment group. However, there were no significant differences in malondialdehyde (MDA) levels and glutathione peroxidase (GSH-PX) activity between the control and treatment groups. Moreover, the number of Vibrio ichthyoenteri was less in the group treated with PMCS (P < 0.05). In conclusion, PMCS can be used to improve the growth performance and health of pacific white shrimp.