The aim of this study was to investigate the effects of adding extra fermented feed on the growth performance, oxidation resistance, immune function and metabolism of juvenile Chinese mitten crabs. Five experimental feeds were prepared: the commercial feed of juvenile crabs was used as the basal feed (F0), the experimental feed was made up of basal feed and extra 5% (F5), 10% (F10), 15% (F15), 20% (F20) fermented feed. 150 Chinese mitten crabs were randomly divided into 5 groups. There were 3 replicates in each group, and 10 crabs in each replicate. All the crabs were fed with the corresponding feed respectively. The feeding period was 59 days. Blood samples and hepatopancreas were collected after culture to determine the antioxidant indexes, immune indexes, and the activities of protein metabolization-related enzyme. The result showed that the weight gain rate and specific growth rate of juvenile crabs in the treatment groups were significantly higher than those in the control group (P < 0.05). The content of catalase in hemolymph first increased and then decreased with the increase of fermented feed, and those in the F5, F10 and F15 groups were significantly increased compared with the F0 (P < 0.05). The activity of acid phosphatase in each treatment group was higher than that in the control group, but the difference was not significant (P > 0.05). The content of total protein and globulin in each group first increased and then decreased with the increase of fermented feed, but the difference among groups was not significant (P > 0.05). In addition, the activities of trypsin, pepsin, aspartate transaminase and alanine transaminase in treatment groups were higher than those in F0. The addition of fermented feed can promote the growth performance of juvenile Chinese mitten crabs, and improve their antioxidant capacity, immune function and protein metabolic capacity.