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• A new Quantitative approach to weight its compositions changes by analyzed functional groups content percentage and to determine its reactivity ranking by analyzed functional groups activation rate. • The chain thermal reaction of ten active groups were analyzed to reveal the oxidation mechanism of functional groups in coal combustion spontaneous. • The inhibition reaction of magnesium chloride on coal samples was different from those of sodium chloride and potassium chloride, which led to the different temperature stages in which the blocking effect occurs. • The inhibition effect of sodium chloride and potassium chloride was mainly manifested in the two-stage involving dehydration/desorption and dynamic equilibrium (below T 3). • The inhibition effect of magnesium chloride presents in the whole coal-oxygen reaction. To study the inhibition characteristics of different chloride salts on the spontaneous combustion of coal and to further reveal why magnesium chloride shows a different inhibitory effect, coal samples from FuSheng Coal Mine No. 15 in Shanxi Province were selected. Based on the characteristics of functional group absorption peak at different temperature, the composition changes and reactivity ranking of the functional groups in oxidized coal samples were tested by thermal oxidation experiments and infrared spectroscopy and analyzed to obtain the chain thermal reactions of different active groups. The weight-loss process and heat release characteristics of raw coal and three chloride salt-containing coal samples (sodium chloride, potassium chloride, and magnesium chloride) were compared and analyzed by synchronous thermal analysis experiments, and their inhibition reaction mechanism were studied. The results showed that different functional groups reacted at different low-temperature stages, and the active groups containing C–H, O–H, C-O, and C = O bonds were important during the spontaneous combustion of coal. In the chain thermal reactions with different active groups, heat and oxidative cleavage of the C-O and O–H bonds determined the chain development of different functional groups, and the R·, RO·, H·, and ·OH, chain thermal reaction products again acted as reactants that promoted further development of the reaction. Sodium chloride and potassium chloride played inhibitory roles in the dehydration-desorption stage and dynamic equilibrium stage through water evaporation and heat absorption, while magnesium chloride participated in the whole coal-oxygen reaction process. The inhibitory mechanism included dehydration and heat absorption by MgCl 2 ·6H 2 O, the complex dehydration of active groups containing O–H, the precipitation and oxygen insulation of [(SH·) n ]Mg and Mg(OH) 2 , the nucleophilic substitution of α-H for RCH 3 and RCH 2 OH groups, and the formation of H...Cl...H bonds. [ABSTRACT FROM AUTHOR]