Microbial flocs is a complex of bacteria, residue of feed, algae, protozoa in aquaculture. Biofloc technology (BFT) based on microbial flocs have been applied in the culture of Oreochromis mossambicus, Litopenaeus vannamei, Macrobrachium rosenbergii and so on. In this study, microbial flocs was achieved by artificial culture in natural marine water. In order to investigate the active ingredients in the microbial flocs, molecular weight, content of total polysaccharide and amino acid in the microbial flocs were analyzed by methods of gel permeation chromatography, Phenol sulfuric acid colorimetry and automatic analyzer for amino acid respectively. The results of preliminary biochemical analysis showed that the average molecular weight of extractive from supernatant of microbial flocs was about 213281Da, the polysaccharide and amino acid content of sediment from microbial flocs was 29.57% and 12.63% respectively. Further study was conducted to investigate the function of microbial flocs to enhance the immune activity of shrimp. Microbial flocs were added in the Litopenaeus vannamei diet at a rate of 0%, 0.02%, 0.1%, 0.5%, and 2.5%. Activity of antibacterial, lysozyme and phenoloxidase (PO) in serum of shrimp were analyzed after 14 days of feeding trial. Challenge test was carried out to evaluate the effect of microbial flocs on improving the ability of shrimp against virulent bacterium of Vibrio Harveyi after 30 days of feeding trial. Results show that activity of antibacterial and lysozyme in serum of shrimp fed diet containing microbial flocs at a rate 2.5% were higher than that in other groups (p<0.05). PO activity in serum of shrimp fed diet containing microbial flocs at rates 0.5% and 2.5% were higher than that in other groups(p<0.05). Challenge test show that cumulative mortality of shrimp fed diet containing microbial flocs at a rate of 0.1% was the lowest than that in other groups(p<0.05). The present results suggested that microbial flocs can be utilized by shrimp and can improve the ability of non-specific immunity of shrimp.