The thermal decomposition process of CL-20/TNT co-crystal in different compression states is simulated by ReaxFF reactive molecular dynamics method. Co-crystals models with different degrees of compression are obtained under four pressures of atmospheric pressure, 4.6GPa, 9.2GPa and 18.4GPa. The potential energy evolution, product formation and cluster evolution in the process are analyzed. We find that the formation of clusters is the key to the decomposition process. When the system is compressed, there will be more clusters which can bind the free atoms, resulting in the reaction to be inhibited. Other evolution information conforms this.