Molecular dynamics (MD) simulation was applied to investigate cyclotetramethylene (HMX) crystal at 295K using COMPASS force field with the isothermal-isobaric (NPT) ensemble. Six models were considered, which were (4×2×4) and (4×4×4) unit cells and the other models obtained by cutting along their (1 0 0) crystalline surface with different depths. The trigger bond length, the interaction energy between two atoms of trigger bond and the mechanical properties of the HMX crystal were analyzed. Results show that the trigger bond length distribution is approximately symmetrical Gaussian distribution and the average bond length (L_ave) is in good agreement with the experimental. The interaction energy (E_N—N) between two N atoms of the N—N trigger bond increases monotonously with the increasing of atom numbers in the models, and the mechanical properties are independent of models except for Cauchy pressure.