To investigate the influence of moisture on the exothermic properties of coal oxidation, this study investigated the variation in thermal effect and radical parameters in the oxidation process of coal under different moisture content. This was achieved through thermogravimetric–differential thermal experiments and electron spin resonance experiments. Additionally, the study analyzed the impact of free radicals on the characteristics of the thermal effect of coal at different oxidation stages using the product–moment correlation method. The results indicate that the moisture content of 8% is a critical point for promoting or inhibiting the oxidation reaction of coal. Below this threshold, it promotes the oxidation reaction, while above it, it plays an inhibitory role. The most significant promotional effect was observed at 8% moisture content, which increased both the weight loss and heat release by 8.61% and 1567.04 J/g, respectively, while also significantly enhancing free radical species and complexity. Conversely, when the moisture content of coal reached 20%, there was a notable inhibition effect, with a reduced weight loss and exothermic capacity by 4.94% and 2705.03 J/g, respectively, along with decreased free radical species and complexity. The free radical species and heat effect parameters in coal showed a strong correlation at all stages of low-temperature oxidation, thus indicating that g-factor can characterize the coal oxidation process to some extent.