The response to temperature of HMX polymorph is important for understanding the sensitivity, stability, and phase transitions of energetic material. Using ReaxFF-lg with isothermal-isobaric molecular dynamics (NPT-MD) methods, the crystal and molecular structures of -, -, and -HMX crystals in a temperature range of 303-503 K and at atmospheric pressure are investigated. The calculated crystal structures and thermal expansion coefficients are in general agreements with experimental results, indicating that ReaxFF-lg potential can correctly describe the thermal expansion of HMX polymorph. The linear thermal expansion coefficients indicate that the thermal expansion of -HMX is anisotropic, the thermal expansion along c axis is slightly different from those for a and b axes for -HMX, and the thermal expansion along a, b, and c axes are almost the same for -HMX. The volume expansion coefficients for the three phases decrease in the following sequence: -HMX-HMXHMX, showing that -HMX is the most sensitive to temperature in the three crystals, which may be the reason for higher sensitivity of this phase. Sharp changes in lattice parameter and molecular conformation transformation from chair to boat-chair occur for HMX when temperature reaches 443 K. Helmholtz free energy derived from the two-phase thermodynamics (2PT) model suggests a phase transition for HMX at T=423-443 K. The , -, and -HMX crystals are stable in the temperature ranges of 303-423 K, 443-503 K, and 363-423 K, respectively.