Abstract: Aiming at the problem that there was a big difference between the critical pressure of hydraulic-fracture coal calculated by the classical elastic-plastic mechanics and the actual situation, the critical pressure of coal failure was studied on the basis of the thermodynamics laws and the energy principle. Firstly, the coal failure criterion was established by adopting the viewpoint of energy release and the formulas of critical pressure at different initiation locations were discussed by mechanics analysis for the water injection process of coal seam drilling. Secondly, taking Pinggang Coal Mine as an example, the critical pressure was calculated by using the energy principle and the classical elastic-plastic mechanics. Finally, the field test results showed that the critical pressure calculated by the energy principle was consistent with the actual site, and the calculation result of the classical elastic-plastic mechanics was relatively larger.