Three-step-two-pot method for synthesizing HMX

The invention discloses a three-step-two-pot method for synthesizing HMX. The method comprises the steps of at first conducting the dissolution reaction of 3, 7-binitro-1, 3, 5, 7-tetraazabicyclic [3. 3. 1] nonane in the system of ammonium nitrate and fuming nitric acid, adding red fuming nitric acid for nitrosation reaction, adding water in dropwise mode, filtering, washing with water, drying to acquire 1-nitroso-3, 5, 7-trinitro-1, 3, 5, 7-tetraazacyclooctane, adding the solution under low temperature into fuming nitric acid, heating for nitrolysis reaction, pouring the solution into ice water, filtering, washing with water, and drying to acquire the target product HMX. The method avoids the use of anhydride acetic acid, ethylic acid, nitric anhydride and ionic liquid and the like, thus reducing cost and pollution to the environment. The acquired product has the advantages of being high in purity, having a yield rate being raised to more than 75%, having the beta crystal form, and avoiding the post-treatment such as oxidation crystallization and crystal phase transformation, and enhancing production safety.