The invention discloses a method for preparing 3-ammoniac methyl-3-methyl trimethylene oxide. The method takes 3-chloromethane radical-3-methyl trimethylene oxide (CMMO) and liquid ammonia as raw materials and comprises an ammoniation reaction and a post-treatment process. The method comprises the following steps: adding the CMMO into a high-pressure reaction kettle, using high-purity N2 to replace system air, using high-purity nitrogen to press the liquid ammonia into the reaction kettle, stirring and reacting for 8-12h at the temperature of 75-90 DEG C and the pressure of 3.2-4.0MPa; after finishing the ammoniation reaction, cooling the materials, taking out the excess liquid ammonia, adding an organic solvent into the reaction kettle, stirring, cooling the materials to 0-5 DEG C, filtering to remove existing crystals, collecting filtrate, evaporating to remove most of the solvent at normal pressure, cooling the materials to 0-5 DEG C again, filtering to remove the crystals in the materials, distilling the filtrate by depressurization and collecting distillates at 42-46 DEG C. A product obtained by the method is an organic intermediate for synthesizing medicines and pesticide.