Crystal quality of explosive affects its processability and safety. The addition of ethyl acetate can effectively improve the surface of HMX crystal in refining process of HMX. The natural volatilization crystallization method was used to study the effect of ethyl acetate content of 0%, 1%, 1.46%, 2.9%, 4.33% and 5% on HMX crystal morphology, crystal form, thermal decomposition properties and crystal quality in acetone-ethyl acetate system. Addition of ethyl acetate less than 4.33%, no change on the morphology and crystal form of HMX, but promotes the β-to δ- polymorphic transition during the decomposition of HMX crystals, and the HMX crystal quality deteriorates. In the research range, when the addition amount reaches 5%, the crystal quality is improved.