Abstract: The introduction of water vapor as a gasification agent in the study of underground coal gasification (UCG) is conducive to the improvement of combustible gas generation, which is of greatsignificance in promoting the advancement of coal clean utilization technology. The effect of the change of vapor-oxygen ratio on the generated gas components during underground coal gasification was investigated, and the effect of the change of vapor-oxygen ratio on the calorific value of coal gas and gasification efficiency (the ratio of calorific value of coal gas to the heat of combustion of carbon) was analyzed through the evaluation of energy recovery. On this basis, the gasification process was divided into an ideal gasification stage and a secondary conversion stage, and the effects of oxygen excess and deviation (the deviation between the actual reaction occurring in the gasification process and the ideal gasification reaction) on the generated gases and calorific values were investigated. The results indicate that changes in the air-to-oxygen ratio significantly affect the volume fraction content of the main combustible components (CO+H₂) in the generated gas. When the steam-to-oxygen ratio increases from 1.5:1.0 to 2.0:1.0, the volume fraction of (CO+H₂) in the generated gas increases with the increase in the steam-to-oxygen ratio, reaching a maximum of 62.39% at a steam-to-oxygen ratio of 2.0:1.0, with the H₂ volume fraction stabilizing at approximately 30%. The gasification efficiency and gas calorific value both reach their maximum values. However, when the steam-to-oxygen ratio exceeds 2.0:1.0, the volume fraction of the main combustible gases begins to decrease. Through staged analysis, it was found that when the oxygen supply exceeds the amount required for the ideal gasification reaction, the combustible gases produced by the reaction undergo a secondary reaction with excess oxygen, generating low-calorific CO₂, thereby reducing the calorific value of the gas. As the deviation increases, the water gas reaction and water gas shift reaction become more intense, leading to an increase in the volume fraction of (CO+H₂), thereby increasing the calorific value. However, when the supply of steam is excessive, the excess steam lowers the reaction temperature, inhibits the water gas reaction, reduces the deviation, and consequently decreases the volume fraction of CO and H₂ in the generated gas, leading to a decrease in calorific value. Therefore, in the experiment, the oxygen excess should be moderately reduced to minimize CO₂ production, while the steam supply should be appropriately increased to enhance the deviation, promote the water gas reaction, and improve gasification efficiency and gas calorific value.