In order to investigate the effect mechanism of protein phosphorylation on apoptosis and myofibrillar protein degradation during maturation, psoas major (PM) muscles injected with protein kinase A (PKA) and alkaline phosphatase (AP) ware used as the experiment subjects. Mitochondrial dysfunction, apoptosis, myofiber type and the degradation of myofibrillar protein were measured and analyzed. The results showed that the AP group had higher mitochondrial membrane permeability, cytochrome c (Cyt-c) oxidation level, and caspase activity at 12~72h postmortem compared with both the PKA and control groups （P<0.05), showing more apoptosis. At 2~48h postmortem, the number of type Ⅰ muscle fiber was significantly increased in both the PKA and AP groups （P<0.05). At 2~72h postmortem, the levels of desmin degradation, troponin-T degradation, and calpain autolysis in the AP group were significantly higher than those in both the PKA and control groups （P<0.05), indicating greater myofiber damage. In addition, the peak activity of caspase-9 occurred earlier in the AP group (at 6h postmortem) than in both the control and PKA groups (at 12h postmortem), and the peak activity of caspase-3 in the PKA group occurred later (at 48h postmortem) than in the AP and control groups (both at 12h postmortem). In conclusion, dephosphorylation induced by AP treatment during postmortem increased mitochondrial functional damage, promoted caspase-mediated apoptosis, and facilitated myofibrillar protein degradation, ultimately exacerbating myofiber damage and improving the tenderness of postmortem meat.