In order to obtain large transparent spherical-shaped RDX crystal, the metastable characteristics of RDX in water-cyclohexanone mixture solvent were studied. Solubilities and super-solubilities of RDX in water-cyclohexanone solution were tested by spectroscopy method, and the metastable zone width of RDX solution was determined. The correlation among temperature, water content and metastable zone width was discussed in detail. It is found that the metastable zone width becomes narrow when temperature rises: the metastable width is 16.4 ℃ in low temperature zone while 15.0 ℃ in high temperature zone when the water content is 8%, and the meta-stable width is 19.1 ℃ and 14.0 ℃ respectively when the water content is 4%. When temperature of RDX solution is above 58 ℃, the metastable width changes more wider when water content increases from 4% to 8%. The large transparent spherical particle RDX crystal, with 2 mm size and smooth surface, is obtained by adding the RDX crystal seed into water-cyclohexanone-RDX solution, which is in metastable state.