To prepare ultrafine and high-quality octogen(HMX) and study its crystal types, superfine HMX was prepared by an ultrasonic assisted spray recrystallization refinement method. The effects of atomization rates, solvent and non-solvent type and temperature etc. on the particle morphology and crystal types of HMX were studied. The crystal morphology, particle size and crystal type of HMX were characterized by scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The thermal decomposition properties of HMX were measured by differential scanning calorimetry (DSC). The impact sensitivity of raw HMX and superfine HMX was measured by an impact sensitivity tester. Results show that the ultrafine HMX prepared by ultrasonic assisted spray recrystallization refining has a great influence on the atomization rates, solvent and non solvent type, temperature etc. The morphology of HMX crystals prepared by an ultrasonic frequency of 40 kHz, stirring speed of 400 r·min^-1, atomization rate of 20 mL·min^-1, acetone as solvent, absolute alcohol as non-solvent and temperature of 30 ℃ is the best, the crystal type is β-type. Compared with the raw material of HMX, the apparent activation energy of thermal decomposition reaction of ultrafine HMX increases by 17. 94 kJ·mol^-1 and its characteristic drop height of impact sensitivity increases by 29. 6 cm.