The mechanical properties of RDX and HMX crystals were investigated by using molecular dynamics simulation method. The fracture properties of the crystals were analyzed with impact test, laser diffraction size analyzer and scanning electron microscope. The results show that the calculated elastic constants and modulus of HMX crystals are higher than those of RDX crystals. The ductibility of RDX crystals is better than that of HMX crystals.Under the same impact condition, both the plastic deformation and fragmentation appear in crystals. The plastic deformation is preponderant in RDX crystals, and the fragmentation is preponderant in HMX crystals and their fracture is much more severe. The molecular dynamics simulation on the mechanical property for different explosive crystals is accuracy and reliability.