1-Nitroso-3, 5, 7-trinitro-1, 3, 5, 7-tetraazacyclooctane (MNX) is an intermediate in the production of 1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetraazacyclooctane (HMX) from 3, 7-dinitro-1, 3, 5, 7-tetraazabicyclo[3.3.1]nonane (DPT) by nitrolysis. To study the mechanism of HMX formation from DPT and fuming nitric acid, formaldehyde was added into the reaction. A new process of preparing MNX by reaction of DPT in red fuming nitric acid (HNO₃/N₂O₄)was investigated. The influence of reaction temperature, loading amounts of nitric acid, N₂O₄ and NH₄NO₃ on the reaction was investigated. The optimum reaction conditions were determined by an orthogonal experiment. The sensitivities of MNX were measured by GJB772A-97 method and WJ/T9038.3-2004 method. Results show that formaldehyde can promote the reaction of DPT and fuming nitric acid to form MNX and the nitrosolysis reaction mechanism in the redox of DPT in nitric acid is proposed.The yield of MNX is 83.5% under the optimized reaction conditions of molar ratio of N₂O₄ to DPT as 1 :1 and molar ratio of NH₄NO₃ to DPT as 2.5 :1 at -25 ℃. Its impact sensitivity and electrostatic sensitivity of MNX are lower than those of RDX and HMX, while the fricon tisensitivity is between RDX and HMX.