The effect of solvent volume ratio of dinitrogen pentoxide/nitric acid/acetic anhydride system on the synthesis of cyclotetramethylenetetramine (HMX) via the nitration of 3, 7-dinitro-1, 3, 5, 7-tetrazabicyclo[3.3.1]nonane (DPT) was investigated. Results show that the highest yield of HMX is 87.9% when the volume ratio of nitric acid/acetic anhydride is 3:2. Raman spectra reveals that, the concentration of nitronium ion in the system gradually decreases with increasing the ratio of acetic anhydride and the significant red-shift of the vibration frequency of nitronium ion occurs, revealing that the trend of the concentration of the nitronium ion with bending deformation in dinitrogen pentoxide ion pairs is increases first and then decreases. The dinitrogen pentoxide which is between the range of certain ion pairs and free ions, as well as certain electron donating effect for solvent system, may have appropriate nitrification ability and high reactiviy, and be conducive to the synthesis of HMX.