Abstract: The microstructure of long flame coal samples subjected to different high temperature steam was studied by means of micro CT technique. The results show that when the pyrolysis temperature is lower than 300 ℃, the loss of free water and gas leads to the formation of cracks. When the pyrolysis temperature is higher than 300 ℃, the interconnecting crack network is formed in the pyrolysis of organic matter, and the pores and cracks are mainly formed in the organic matter. Pyrolysis can be divided into three stages. Before 300 ℃, coal samples are pyrolyzed slowly, and pores and cracks increase slowly. The pyrolysis intensifies between 300 ℃ and 500 ℃, and the porosity of coal samples increases obviously. When the pyrolysis temperature is higher than 500 ℃, the pyrolysis is the most complete and the porosity of coal samples increases sharply. After 550 ℃, the volume porosity of the coal sample reaches 32.35%, which is 9.82 times higher than that at 25 ℃. Moreover, 300 ℃ is the threshold point of pyrolysis. When the pyrolysis temperature is higher than the threshold point, different bedding planes are well developed, forming a pore network through each other, accelerating pyrolysis and providing a migration channel for pyrolysis products.