Abstract: To explore the mechanism of Chinese mitten crabs (Eriocheir sinensis) dealing with adverse water environmental stresses, we studied the effects of nano cerium oxide (nCeO₂) on the immunity and antioxidant capacity of the crabs under ammonia-nitrogen and Aeromonas hydrophila stresses. The crabs were continuously fed with diets supplemented with different doses of nCeO₂ (0, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4 and 12.8 mg·kg⁻¹) for 90 d, then the ammonia-nitrogen and A. hydrophila stress tests were conducted. The results show that: 1) Both the single ammonia-nitrogen stress and dual stresses of ammonia-nitrogen and A. hydrophila (henceforth called "dual stress") decreased the survival rates of the crabs significantly (P<0.01); the activities of acid phosphatase (ACP), alkaline phosphatase (AKP) and lysosome (LZM) increased (P<0.01 or P<0.05); the activities of superoxide dismutase (SOD) and catalase (CAT) decreased significantly (P<0.01), while the concentration of malondialdehyde (MDA) increased (P<0.01 or P<0.05); 2) Suitable nCeO₂ dose could enhance the survival rates of crabs under the single ammonia-nitrogen stress and dual stress (P<0.01 or P<0.05), increase the activities of SOD and CAT significantly (P<0.01), decrease the concentration of MDA (P<0.01 or P<0.05), further increase the activity of LZM (P<0.01), but the activities of ACP and AKP only increased under dual stress (P<0.01). Above all, suitable supplementation of nCeO₂ can ameliorate the decline of survival rates and antioxidant capacity of E. sinensis under the the single ammonia-nitrogen stress and dual stress, and increase the immunity enzyme activities. The suitable nCeO₂ supplementation doses for protecting crabs are 0.8 and 1.6 mg·kg⁻¹ under single and dual stress, respectively.