Abstract: In order to investigate the impact of microscopic pore structure of coal to its spontaneous combustion tendency, taking gas coal, fat coal and coking coal samples as the research objects, the microscopic pore distribution data of different coal samples were measured by using the mercury injection experiment. The results showed that pore fractal dimension was between 2 and 3 when the diameter less than100 nm, the coal fractal characteristics of microscopic pore was good, meeting th e fractal theory to describe the microscopic pore distribution properties of porous medium. The temperature programmed oxidation simulation experiment of gas coal, fa t coal and coking coal samples were done by coal spontaneous combustion simulation system, the curve of gas product concentration with temperature variation was dr awn, the experimental results showed the relationship between microscopic pore structure and its spontaneous combustion tendency: the metamorphic degree of coal s amples were higher, its internal structure was compact, endogenous fracture was relatively small, the temperature of the oxidation reaction was also higher.