Walnut peptides (WP) were prepared from walnut protein. Single-factor experiments and Box–Behnken response surface methodology were used to optimize the preparation of walnut peptide–calcium chelate (WP–Ca), followed by evaluation of its structural characteristics, stability, and in vitro digestion properties. The optimized conditions were a WP concentration of 27.8 mg/mL, reaction temperature of 55.7 °C, pH 7.9, reaction time of 10.1 min, and WP-to-CaCl₂ mass ratio of 3:1, giving an experimental Ca²⁺ chelation rate of 54.21%. Chelation altered the amino acid composition, particle size, functional group environment, molecular organization, microstructure, and surface charge of the sample. WP–Ca maintained relatively high calcium retention rates under neutral and low-temperature conditions. Calcium release rates during simulated gastric and intestinal digestion were 81.90–92.38% and 39.05–48.57%, respectively, and WP–Ca retained antioxidant activity. These findings provide a basis for developing walnut peptide–calcium nutritional products.