Dynamic changes in the in vitro digestive properties of postmortem mirror carp (Cyprinus carpio L.) at various intervals (0, 4, 24, 72, 120, and 168 h) under refrigerated conditions (4 °C) were evaluated based on protein degradation and oxidative reactions. Results indicated that protein degradation and oxidation intensified over storage time, as evidenced by increases in total viable counts, β-sheet content, and surface hydrophobicity, alongside decreases in α-helix content and fluorescence intensity (p <
0.05). The myofibril fragmentation index and TCA-soluble peptide levels peaked at 72.90 and 6.91 μmol tyrosine/g, respectively, whereas the total sulfhydryl content dropped to 50.25 μmol/g after 168 h postmortem. In vitro simulated gastrointestinal digestion experiments further demonstrated that aging enhanced protein digestibility. This study elucidates the postmortem dynamic patterns of freshwater fish, providing a theoretical basis for quality regulation.