Abstract: The purpose of this paper is to explore the change law of coal quality characteristics of different metamorphic degree coals in the process of low temperature oxidation. Through the coal quality analysis index measuring system and firmness coefficient measuring device, the changing rules of coal quality analysis index and firmness coefficient of coal with different metamorphic degrees during low temperature oxidation were studied. The interaction between coal quality analysis index and firmness coefficient is explored. The results show that the lower the metamorphic degree of coal is, the more delayed the evaporation of internal water in the stage of low temperature oxidation is, and the easier the volatile matter is to volatilize and decompose. With the increase of oxidation temperature, the f values of the three groups of coal samples decreased continuously. The effect of water evaporation on the firmness coefficient is mainly reflected in the early stage of low temperature oxidation, and in the later stage of low temperature oxidation, the development of cracks caused by volatile matter volatilization results in the decrease of firmness coefficient. For lignite, the decreasing rate of f value keeps increasing in the middle stage of low temperature oxidation, which is mainly due to the joint effect of water evaporation and volatile decomposition.