Polymer-bonded explosive (PBX), also known as plastic-bonded explosive, is a typical kind of explosive powder with the synthetic polymer bonded together explosive composite materials. It has excellent explosion performance and thus is widely applied in the military and civilian industries. The PBX mechanical properties exhibit high sensitivities to the action of various types of loads, which is closely related to microscopic damage mechanisms within the material. The applied loads vary considerably, with amplitudes ranging from a few MPa to as high as several tens of GPa and durations lasting from the order of μs to ms. The cracking evolution is essential to the PBX applications in a strict control manner. However, PBXs are dangerous energy-bearing materials, and the mechanical experiments to measure their mechanical properties are costly and also challenging. Therefore, theoretical analysis and numerical simulation are anticipated to explore the sensitivities of PBX mechanical properties and also the influence of crack evolution. This paper will simulate numerically the process of work done of PBX explosive gas by fracture phase-field method, which reveals the typical microscopical mechanism of crack evolution and establish a computational model for crack propagation under the coupled thermomechanical effects.