In order to investigate the effects of stocking density on the immunity, growth and water quality of Fenneropenaeus merguiensis cultured under biofloc system, Fenneropenaeus merguiensis with average body mass of (2.46±0.39) g was selected for test, stock density groups of 100 ind/m3, 300 ind/m3 and 700 ind/m3 were set, with 3 replicates in each group, and no organic carbon source was added and no water was changed during the test of 30d. The results showed that the stocking density had an effect on the immune indexes of different tissues of Fenneropenaeus merguiensis: with the increase of stocking density, in hepatopancreas,  ACP activity increased gradually, AKP activity decreased gradually, and CAT and SOD activities first increased and then decreased; in muscle, SOD and ACP activities first decreased and then increased, and CAT activity decreased gradually; there was a significant difference in the growth traits of shrimp (P<0.05), the growth of body mass and the specific growth rate of body mass decreased with the increase of density, and the survival rate also decreased; the water quality indicators also showed different trends: with the increase of density, the variation of water quality indicators increased, and the stability of water quality was poor. The study showed that under the biofloc culture mode, the stocking density significantly affected the immune-related enzyme activity, growth and water quality of Fenneropenaeus merguiensis.