Abstract: The desorption process of coal is interpreted from the perspective of thermodynamics. The four pending parameters of temperature-pressure-adsorption equation (TPAE) were obtained by using the measured series isothermal adsorption data of high-order structural coal and primary coal. The TPAE is transformed into an indefinite integral formula similar to the Clausius-Clapeyron equation, and an isosteric adsorption enthalpy is obtained. The product of the firmness coefficient and the desorption threshold is defined as the outburst threshold (OT), and is used to compare the difficulty of the coal and gas outburst between several types of coal. The results show that adsorption is a spontaneous heat release process, and desorption is not a spontaneous heat absorption process. The external energy required for desorption is equivalent to the threshold for desorption to occur. The desorption threshold (abbreviated as DT) is the unit isosteric desorption enthalpy. Through calculation, the occurrence of structural coal is 8.1 times more likely of coal and gas outburst than primary coal. The desorption rate of structural coal is no-linearly decreased with unit time. The greater the external energy, the faster the desorption rate decays. Blasting produces a large amount of external energy, immediately beyond the OT. It is, therefore, easier to induce coal and gas outburst.