In this work, we develop an NNP to explore the decomposition mechanism of RDX crystals. The NNP is first trained and validated against the DFT database with bulk RDX molecules under a wide range of thermodynamic states. The lattice constant, the equation of state, decomposition rate, and reaction pathways are calculated with the NNP to evaluate its accuracy. Finally, the decomposition mechanism of bulk RDX molecules is proposed for the first time at the ab initio level of accuracy.