As a key precursor in the preparation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 3,7-dinitro-1,3,5,7-tetraazabicyclo[3,3,1]nonane (DPT) prepared from urea by a modified one-pot method has a bright future. On the basis of our previous work (Reaction Mechanism of One-Pot Synthesis of Dinitro Pentamethylene Tetramine, Chinese Journal of Organic Chemistry 2010, 30 (3), 414-418.) [1], a novel intermediate 3-nitro-1,3,5,7-tetraazabicyclo[3,3,1]nonane was captured and isolated, and the key reactive intermediate hexahydro-1-nitro-1,3,5-triazine was directly captured and confirmed by HPLC-MS. Thus, a modified synthetic mechanism for DPT was elaborated, and based which DPT preparation process was optimized.