Agaricus bisporus tastes good and is nutritious, but its respiratory metabolism is strong and consequently its shelf life is short. In order to solve the problem of its poor storability, Agaricus bisporus was stored for a short period of time under oxygen-free modified atmospheres containing different levels of CO₂ (5%, 10% and 20%) or a pure nitrogen atmosphere as a control and evaluated for postharvest quality indicators (mass loss, lightness, browning degree, texture, and taste value) and physiological indicators (respiration rate, relative conductivity, malondialdehyde (MDA) content, antioxidant contents). We found that a low-CO₂ (5%) atmosphere was the most effective in preserving the quality of Agaricus bisporus, which could reduce the respiration rate and mass loss, inhibit browning, maintain firmness and springiness, delay the increase of cell membrane permeability, reduce oxidative damage and maintain the umami taste of Agaricus bisporus. But the preservation effect of a high-CO₂ atmosphere was relatively poor, and the mushroom stored under a 20% CO₂-containing atmosphere showed serious browning, increased cell membrane permeability and consequently serious electrolyte leakage, accompanied by oxidative damage.