A mesoscopic mechanics model of random distribution of explosive particles was constructed by applying circular approximation. The numerical simulation of mechanical behaviors in a pressing process of HMX based polymer bonded explosive (PBX) based on the material point method was carried out. The change in stress and temperature of the system and the deformation of explosive particles during the pressing process were emphatically examined. Simulation results show that the compression process can be divided into two stages: integration stage and consolidation stage. In the integration step, the explosive particles are subjected to pressure and rearrangement. The stress is produced at the contact part between explosive particles and compression surface, forming several stress trains and then the stress trains is propagated along the contact surface between explosive particles upward and downward. Temperature in the system gradually increases in the stress process. The maximum temperature rise in the locality of the system is 10 K. In the consolidation step, after the plastic deformation process of the system, the internal stress gradient decreases, at the same time, obvious temperature difference in the system is existed, the maximum temperature difference is 20 K.