Abstract: The heat output of coal, the rate of oxygen consumption and the influence function of coal size were tested by a big coal spontaneous combustion experiment unit. The relationship between the coefficient of heat convection and air flux of roadway was studied. Based on the conservation principle of energy and the real conditions, such as coal temperature, air temperature, geometry size of roadway, air flux, oxygen concentration in the loose coal around the roadway etc, the terminal parameters resulting in coal spontaneous combustion are proposed, the conditions and the method for determining coal spontaneous combustion are set up, and the quantitative index determining the danger zone of coal self-ignition is obtained.