In this study, we numerically investigate the simultaneous thermodynamic and hydrodynamic mechanisms of underwater explosion (UNDEX). Bubble explosion in water at the collapsing stage is extremely violent and becomes extraordinarily hot, exceeding 1,000 K. The evolution of the bubbles and temperature fields are simulated using a fully compressible mixture model. The deformable bubble and the heat transfer of the internal explosive gas are captured with higher accuracy compared with published data. First, a spherical bubble that collapses and rebounds without the effects of gravity is computed to verify the accuracy of the model. The numerical results in terms of the bubble radius and temperature fields are consistent with analytical solutions based on the Rayleigh–Plesset equation. Next, a real 5.2 g trinitrotoluene UNDEX experimental case is simulated, in which the formation of a non-spherical bubble with a non-symmetric thermal boundary layer is analyzed. An excellent agreement between bubble motions and experimental data is obtained. The temperature inside the collapsing bubble increased significantly, reached a maximum value of approximately 2,000 K at its final stage, and then decreased rapidly. In addition, a spatially non-uniform temperature field and a thicker thermal boundary layer along the jet direction at the collapse stage are observed. Furthermore, a case study is conducted to estimate the bubble dynamics of non-isothermal and isothermal cases. Finally, the effects of the initial equilibrium gas temperature and water temperature on the thermodynamic and hydrodynamic mechanisms of UNDEX are investigated in detail. An approximate non-linear relation is proposed to describe the relationship among the important parameters.