Abstract: Semi-aerobic landfill was set up.During the steady stage of landfill gas production,the concentrations of CH₄,CO₂ and O₂ in different points were detected,and the correlativity between O₂ concentration and CH₄,CO₂ concentrations were simulated with mathematical model.The results showed that: with O₂ concentration increasing,the CH₄ concentration declined linearly and the CO₂ concentration declined exponentially in upper layer;the concentrations of CH₄ and CO₂ both declined exponentially in middle layer;in bottom layer the CH₄ concentration was declined exponentially,but that of CO₂ kept fluctuated around 34% initially,and then declined exponentially.The CH₄ concentration was lower than that of CO₂ in upper layer;in middle layer the CH₄ concentration was higher than that of CO₂ when O₂ concentration was less than 5.8%,the CH₄ concentration was lower than that of CO₂ when O₂ concentration was more than 5.8%;In bottom layer the CH₄ concentration was higher than that of CO₂ when O₂ concentration was less than 2.8%,the CH₄ concentration was lower than that of CO₂ when O₂ concentration was more than 2.8%.