Microspherical and desensitized HMX was prepared by pneumatic spray thinning method. The crystal shape,particle size and thermostability of ultrafine HMX were analyzed by scanning electron microscope and differential scanning calorimetry(DSC),and the impact sensitivity was measured by impact sensitivity analyzer. The results show that: the microspherical and desensitized HMX with average particle size of 450 nm was prepared when pressure was 0.5 MPa,and water temperature was 10 ℃,and concentration was 0.2 g·mL^-1,and volume ratio for water to the solution was 80/1. The thermostability of ultrafine HMX is enhanced by 1.0% compared to that of the initial HMX,and the impact sensitivity of ultrafine HMX is decreased by 87% compared to that of the initial HMX.