The octahedral cyclotrimethylenetrinitramine crystal （O-RDX） with an average particle size of 9.35 μm was prepared through the solvent-antisolvent method in the dimethylsulfoxide （DMSO） -ethylene glycol （EG） system using 1-ethyl-3-methylethimidazole acetate （EmimOAc） as an additive. The effects of crystallization parameters such as solvent system， solution concentration， crystallization temperature， additive and stirring speed on the growth behavior of RDX（cyclotrimethylenetrinitramine） crystals were systematically studied. It was observed that the main factor influencing the growth of RDX crystals was the supersaturation. With the gradual decrease of supersaturation， RDX crystals experienced the changes of rough growth， 2D growth and spiral growth， and the morphology of RDX crystals gradually evolved from dendritic to octahedral crystals. The results of analytical tests revealed that O-RDX crystals were in the α-form which was consistent with the raw RDX， showing high crystal density with few crystal internal defects and an increase of 5 ℃ in decomposition temperature. Moreover， compared to the raw RDX， the impact sensitivity and the friction sensitivity of O-RDX decreased by 60% and 50%， respectively. To further explore the formation mechanism of O-RDX， adhesion energy model and the molecular dynamics method were applied to simulate the crystal morphology of RDX in the presence of EmimOAc. The simulated results demonstrated that there were six main crystal faces of RDX： （1 1 1）， （2 0 0）， （1 0 2）， （0 2 0）， （2 1 0）， and （0 2 1）. The formation of the double-cone octahedral morphology originated from the uniform growth rates of the main crystal faces of RDX under the action of EmimOAc. The theoretical simulations generally agreed well with the experimental phenomena.