Agaricus bisporus (A. bisporus) is highly susceptible to postharvest quality deterioration, including enzymatic browning, cap opening, water loss, and tissue softening. Carbon dioxide (CO2) modified atmosphere packaging is effective in maintaining storage stability of post-harvest edible mushrooms. This study aimed to explore the mechanism by which 6 % CO2 modified atmosphere packaging maintains the postharvest quality of A. bisporus. Postharvest A. bisporus were treated with 6 %, 12 %, 18 % CO2 or air (CK), and stored at 4°C for 18 days. Results showed that 6 % CO2 significantly delayed quality deterioration by coordinating energy metabolism and Ca2+ signaling. Specifically, the treatment enhanced mitochondrial respiratory chain activity, increasing ATP content and energy charge (EC), thereby sustaining the energy supply of postharvest A. bisporus. Meanwhile, 6 % CO2 upregulated antioxidant enzymes, thereby suppressing reactive oxygen species (ROS) accumulation and reducing lipid peroxidation. In addition, it maintained cellular architecture by inhibiting phospholipase and cell wall hydrolase activities. Furthermore, 6 % CO2 elevated cytosolic Ca2+ levels and calmodulin (CaM) content in A. bisporus, while upregulating CML and CaMK expression. This activated Ca2+/CaM/CaMK pathway modulates cell membrane and cell wall metabolism by enhancing ROS scavenging capacity. The pathway likely also coordinates cellular energy homeostasis through downstream signaling. Collectively, these results indicate that 6 % CO2 delays postharvest quality deterioration of A. bisporus via the synergistic regulation of energy metabolism and calcium signaling, providing a mechanistic framework for understanding postharvest quality management of this species.