<p>Magnesium (Mg) in feed or water is critically important for the growth and molting of crustaceans. The aim of this study was to determine the optimal water Mg concentration for the aquaculture of Chinese mitten crabs (<i>Eriocheir sinensis</i>) (initial body weight: 30.11 ± 0.93&#xa0;g). Six experimental groups (M0, M15, M30, M45, M60, and M75) were established with varying Mg levels for eight weeks to evaluate growth, Mg deposition, antioxidant capacity, immunity and molting performance. A total of 240 male crabs with intact appendages were randomly allocated to 6 treatment groups (4 replicates per group, 10 crabs per replicate). Results showed that the M30 group exhibited superior growth, molting and immune responses (<i>P</i> &lt; 0.05). Meanwhile, a significant upregulation was observed in genes related to the antioxidant and non-specific immune defense systems and the molting signaling pathway (<i>P</i> &lt; 0.05). While Mg deposition in gill and hepatopancreas tissues significantly increased in the M60 and M75 groups (<i>P</i> &lt; 0.05), excessive Mg²⁺ concentrations (M60 and M75) inhibited growth, molting and immune function and caused tissue damage. Additionally, Mg affected ecdysteroid and molt-inhibiting hormone levels in hemolymph (<i>P</i> &lt; 0.05). The ‌broken-line model‌ and quadratic regression model analysis identified ‌33.84–37.37&#xa0;mg/L Mg²⁺‌ as the optimal concentration for culturing <i>E. sinensis</i>. This research identifies the optimal Mg²⁺ concentration for <i>E. sinensis</i> aquaculture industry, establishing a scientific foundation for future mineral nutrition studies.</p>