Abstract: With the increasing influence of human activities on the climate, global warming and a variety of thermal pollution become more and more serious, which leads to the aggravation of fish disease outbreaks worldwide and thus causes serious production and economic losses to aquaculture and fishery industries. Moreover, with the rapid development of aquaculture industry, the abuse of antibiotics has caused a series of problems such as the deterioration of aquaculture environment, frequent occurrence of diseases and drug residues, which not only hinder the sustainable development of aquaculture industry, but also pose a serious threat to human health. Therefore, the research and application of antibiotic substitute additives have attracted great attention. Polysaccharide from Traditional Chinese Medicine (TCM), having a variety of biological activities and no toxic and side-effect, is a green, safe and non-residual feed additive with great potential for development and is potential to be used as an antibiotic substitute product. Yu-Ping-Feng powder is a compound Chinese herb medicine composed of Atractyloids, Astragalus and Fangfeng and Yu-Ping-Feng Polysaccharide (YPF-P) has the strongest pharmacological activity among the polysaccharides in YPF-powder. It has the effect of enhancing aquatic animal immunity, improving intestinal morphology and structure and promoting growth and development. Probiotics refer to microorganisms that promote other biological health, such as Lactobacillus acidophilus, Streptococcus thermophilus and Bacillus subtilis, etc. They are relatively stable in an acidic environment of the intestinal tract of fish, which can not only improve the immunity of the organism but also promote the level of glucose metabolism in the fish. Grass carp is one of the four major fish, and is the largest economic fish in freshwater aquaculture production in China, which has the characteristics of rich nutrition and delicious taste. Hence, in this study, the effect of polysaccharide additive of Traditional Chinese Medicine was improved by preparing the YPF-P compound synbiotics, and its effect on the immune function of grass carp under high temperature was studied, so as to lay a research foundation for the application of YPF-P compound synbiotics in grass carp as an antibiotic substitute additive. To investigate the effects of Yu-Ping-Feng polysaccharide Synbiotics on immune performance of grass carp under heat stress, 360 healthy grass cars weighed of (20±2) g were randomly divided into four groups with 3 replicates per group and 30 fish per replicate for 21 days. The 3 experimental groups (group Ⅰ, Ⅱ and Ⅲ) were supplemented with YPF-P at 0.08%, 0.12% or 0.20% in 0.1% Synbiotics (including Streptococcus thermophilus, Lactic acid bacteria and Bacillus subtilis). At the end of experiment, the grass carps were treated with heat stress various time points before collecting samples. The results showed that 1) in each group, under stress, the activity of the Alanine transaminase (ALT) increased at the beginning and dropped with time in the later period; the activity of Aspartate Transaminase (AST) displayed an upward trend and were different between different time points (P<0.05). 2) In each group, the activities of Superoxide dismutase (SOD), Glutathione peroxidase (GSH-Px) and Glutathione reductase (GR) and Total antioxidant capacity (T-AOC) showed an upward trend at the beginning and reduced in late period; the content of Malondialdehyde (MDA) increased by stress. At any time points, the activities of the antioxidant enzyme (SOD, GSH-Px and GR) in the group Ⅲ were lower than the other two groups (group Ⅱ and Ⅳ). 3) In each group, stress induced the expressions of Nrf2, Keapl, HSP70 and IL-8 reduced the expressions of Cu/Zn-SOD, GR and MHC Ⅱ in a time-dependent pattern; the expression of GSH-Px at three time points under stress was not different from that before stress (P>0.05), but it displayed downward trend over time. These results indicate that grass carp can produce a strong stress response after heat stress, and 0.12% Yu-Ping-Feng polysaccharide compound Synbiotics can improve the anti-oxidative stress ability, effectively relieve the oxidative damage of liver, and have a preferable immune regulation effect.