Abstract: A series of coal bodies with different foreign moisture was prepared by soaking a long flame coal with low water content. The effects of foreign moisture on the rate of temperature rising and the release of CO and CO₂ were studied by simulating coal spontaneous combustion. Moreover, the action mechanism of foreign moisture on the spontaneous combustion of coal was discussed in combination with the characterization of thermal analysis and the pore structure. The action regime of foreign moisture varies with the development of coal spontaneous combustion. At the slow oxidation stage, the foreign moisture plays a physical inhibition effect on coal spontaneous combustion mainly by isolating the contact reaction of coal and oxygen. At the accelerated oxidation stage, the water may involve in the process of the coal-oxygen reaction, promoting the rate of coal self-heating. After entering the rapid oxidation stage, due to the large amount of external water removal during the first and second stages, the pore structure and specific surface area of the coal body are increased; the higher the moisture content of coal, the more active sites will be generated, and the external water presents an indirect promoting effect. When the coal body temperature reaches 180 ℃, with the further increase of coal body temperature, the spontaneous combustion characteristics of coals with different initial external moisture content gradually tend to be consistent.