Abstract: In view of the problem that the exothermic reaction of polyurethane materials for mine may cause underground fire, the possible causes of underground fire were analyzed by means of heating oxidation experiment and thermal decomposition experiment. The results show that there are two causes for underground fire: the external fire that the inside of polyurethane accumulates much heat and react with oxygen through cracks; the internal fire that the heat from polyurethane accelerates the spontaneous combustion of surrounding coal. Other underground fuel such as wood, bamboo and cotton can’t ignite the coal and polyurethane directly, but may accelerate the process of combustion. CO will be produced by polyurethane oxidation, so it is necessary to strengthen downhole fire prediction.