Abstract: CO released from the oxidation of residual coal in goaf areas causes CO exceedance in the return air corner of coal mines, and clarifying the critical thresholds for CO exceedance in the return air corner and establishing a graded early warning index system are of great significance for the early warning of coal spontaneous combustion in goaf areas. To achieve accurate early warning of coal spontaneous combustion disasters in coal mine goaf areas, a medium-rank bituminous coal was taken as an example, and the oxidation characteristics of coal and the generation patterns of indicator gases under ambient temperature and heating conditions were systematically analyzed by combining laboratory experiments and field observations. Experimental results indicated that coal exhibited a hysteresis phenomenon of O₂ consumption and CO generation at ambient temperature, and the background CO volume fraction generated by ambient-temperature oxidation was determined to be 18×10⁻⁶. During the heating oxidation stage, the oxygen consumption rate and CO generation rate showed a significant increasing turning point with a sharp increase at 70 ℃, and the characteristic gas C₂H₄ began to appear at 100 ℃. Based on these results, the low-temperature oxidation process of coal was divided into three stages, namely slow oxidation (30–70 ℃), accelerated oxidation (70–100 ℃), and intense oxidation (>100 ℃). By integrating experimental data with historical field monitoring data, a four-level early warning index system for coal spontaneous combustion was established with the CO concentration in the return air corner as the core indicator, which provided a theoretical basis and practical guidance for early identification and graded prevention and control of coal spontaneous combustion based on CO concentration in the return air corner.