To further reveal the effect of cocrystallizing and mixing on the sensitivity, mechanical properties and thermal decomposition mechanism of hexanitrohexaazaowuetzitane (CL-20)/1, 3-dinitrobenzene (DNB) energetic material from molecular level, the sensitivity, binding energy and mechanical properties of CL-20, DNB, CL-20/DNB cocrystal and CL-20/DNB mixture were simulated and calculated using molecular dynamics (MD) simulation method under the condition of COMPASS stress force field. Their thermal decomposition mechanisms were studied under the condition of ReaxFF/lg force field. Results show that the cocrystallizing and mixing can reduce the sensitivity of CL-20, increase that of DNB, but the cocrystallizing effect is more obvious. Compared with the mixture structure, the cocrystallizing structure is more stable. Cocrystallizing and mixing can change the mechanical properties of the system, reduce the stiffness, increase the flexibility and safety of CL-20 system, but mixing makes the mechanical properties of the system deteriorate. The strong effect between cocrystal molecules can promote the thermal decomposition of the components in the system. NO₂, N₂, NO, H₂O, HONO, HON and CO₂ are the main products of cocrystallizing and mixed systems. Considering that compared with mixing, co-crystallization is a mor effective modification way for energetic materials. which can provide theoretical guidance for the formulation design of energetic materials.