Abstract: In order to investigate the reaction mechanism of moisture in coal acting on coal spontaneous combustion process, the isotope labeling method was innovatively introduced and the migration and transformation rule of each element in H₂O was discussed by using the principle of element tracer. The experimental coal samples were soaked, dried and soaked with three kinds of isotopic moisture (distilled water, D₂O and H₂¹⁸O) to obtain different forms of isotope-labeled moisture coal samples. The process of coal spontaneous combustion was simulated by temperature-programmed oxidation experiment, and the volume of gas product was measured by gas chromatography. Meanwhile, the ion current intensity of isotope atoms participating in the generation of various gases was measured by online mass spectrometer, and the migration rule and formation mechanism of hydrogen and oxygen atoms acting on gas formation in H₂O were studied. Isotopic gases such as C¹⁸O₂, C¹⁸O, and CH₃D were detected in the gas products of coal spontaneous combustion, which indicates that H and O atoms in H₂O can directly participate in the reaction of coal spontaneous combustion. Different forms of moisture in coal play a role in different stages of coal spontaneous combustion reaction: external moisture begin to participate in the reaction at the start of the slow oxidation stage of coal spontaneous combustion reaction, and weaken in the later stage of rapid oxidation. The intrinsic moisture participate in the accelerated oxidation stage and the rapid oxidation stage, and the effect is the strongest at around 140 ℃. H atom and O atom in H₂O have different pathways of taking part in the chemical reaction of coal oxygen. H atoms react with methyl radicals in coal molecules to generate CH₃D, while O atoms can participate in the generation of various carbonyl-containing compounds.