The cocrystal of ammonium dinitramide(ADN)/18-crown-6(18C6) was prepared by solvent evaporation method, and its crystal structure was determined by X-ray single crystal diffraction(SXRD). The hygroscopicity of cocrystal was measured by weight gain method. Results show that the cocrystal belongs to monoclinic system, the space group is C₂/c with the unit cell parameters of a=23.935 (3) Å, b=8.6327 (11) Å, c=20.324 (3) Å, β= 112.874 (3)°, Z=8. The formation of the cocrystal mainly depends on medium strong N—H…O hydrogen bonds action. The formation of ADN/18C6 cocrystal structure makes the moisture absorption rate of AND reduce from18% to 1.2%.