Abstract: Density stress has a negative impact on the immune and antioxidant functions of aquatic animals. In order to improve the potential production of mud crab (Scylla paramamosain) and provide a theoretical basis for its aquaculture development, we cultured the mud crabs for 72 h at three different densities: 8 ind·m⁻² (Low density group), 16 ind·m⁻² (Medium density group), and 32 ind·m⁻² (High density group) to study the effects of density on the antioxidant and immune capacity of mud crabs. The results of antioxidant enzyme activity show that the activities of catalase (CAT) and superoxide dismutase (SOD) in the high density group were significantly higher than those in the low density and medium density groups (P<0.05), but there was no significant difference in the glutathione peroxidase (GSH-Px) activity among the groups. Additionally, the malondialdehyde (MDA) content in the high density group was significantly higher than that in the low density and medium density groups. The activities of acid phosphatase (ACP), alkaline phosphatase (AKP) and lysozyme (LZM) in the high density group were significantly lower than those in the low density and medium density groups (P<0.05), but there was no significant difference between the medium density group and the low density group. The real-time PCR results indicate that the heat shock protein 70 (HSP70) expression level in the high density group decreased first and then increased, significantly lower than that in low density and medium density groups from 6^th hour to 48^th hour (P<0.05). The expression level of Caspase 3 in the high density group increased significantly after 6 h and decreased gradually after reaching the maximum level at 24^th hour, but was always significantly higher than that in the low density group (P<0.05). In conclusion, high density culture can cause oxidative damage, leading to the decline of immunity of mud crabs. The stock density of 16 ind·m⁻² has no significant effects on the antioxidant and immune abilities of mud crabs.