The structures and intermolecular interactions for octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine (HMX)/N, N-dimethylformamide (DMF) solvate have been investigated through quantum chemistry calculations and molecular dynamics (MD) simulations. MD simulation for a β-HMX molecule immersed in DMF solvent shows that β conformation has been transformed into α form. Theoretical calculations at the MP2/6-31G^* level also indicate that the α-HMX is more stable than β-HMX in DMF solution. This explains why all HMX molecules present α-form in the polymorphic forms of HMX/DMF solvate. Geometrical optimizations at the level of MP2/6-31G^* are performed for all the possible homodimers and heterodimers between α-HMX and DMF. Results reveal that C—H…O hydrogen bond interactions exist between components. Moreover, the stabilization energy of α-HMX/DMF is very close to that of α-HMX/α-HMX and much larger than that of DMF dimer. This means that the heteromeric intermolecular forces can compete with the homomeric intermolecular forces, and co-crystallization is possible to occur from the thermodynamics viewpoint. MD simulation for the supersaturation solution of α-HMX in DMF presents the intermolecular interactions is in favor of co-crystallization. These theoretical investigations provide valuable information for understanding why there occurs co-crystallization other than re-crystallization in DMF solution of HMX.