The phase transitions and mechanical properties of the energetic material octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) in three pure polymorphs crystals, alpha-, beta-, and delta-phases, have been studied theoretically. isothermal isobaric molecular dynamics (MD) simulations were performed in the temperature range of (5 to 555) K at 0 GPa and the pressure range of (0 to 40) GPa at 298 K. The lattice parameters and volumes show nonlinear dependence on the temperature and pressure. In the high-temperature regime, there exist two phase transitions, that is, from beta- to alpha-HMX at 360 K and from alpha- to delta-HMX at 440 K. Under high pressure, the beta-HMX transition to delta-HMX is shown at 27 GPa and 298 K. Within the range of temperature and pressure studies, it can be deduced that the beta-HMX at lower temperatures and higher pressures, the alpha-HMX at higher pressures, and the delta-HMX at lower temperatures have better malleability.