CH₃NH₂·HCl was used to replace NH₄NO₃ as an additive in the reaction of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane (DPT) in fuming nitric acid, and the effect of CH₃NH₂·HCl on the formation of 1-nitroso-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane (MNX) was investigated. The process of preparing MNX by CH₃NH₂·HCl promoting the reaction of DPT with fuming nitric acid was studied. The effects of loading amounts of fuming nitric acid and CH₃NH₂·HCl, reaction temperature and reaction time on the reaction were investigated. Results show that the optimum reaction conditions determined by orthogonal experiments are： n (DPT)∶n(CH₃NH₂·HCl)=1∶2.5，reaction time 10 min and reaction temperature -25 ℃. Under the optimum reaction conditions, the yield of MNX is 78.5%. This method avoids using NaNO₂ solution and N₂O₄ as nitroso resource and simplifies the preparation process of MNX. The waste liquid is easy to be treated. The recovery of nitric acid is 75%. CH₃NH₂·HCl can promote the reaction of DPT with fuming nitric acid more significantly than NH₄NO₃. A possible mechanism of promoting the reaction of DPT and fuming nitric acid via decomposing CH₃NH₂·HCl (or NH₄NO₃) into CH₃NH₂ (or NH₃) as a Lewis base catalyst is proposed, which involves the process of nitrous acid formation from nitric acid through a redox reaction as nitroso resource.