Abstract: The variation of strain rate,dynamic mechanical properties and deformation and failure characteristics of coal / rock samples under different impact velocities were investigated by the Split Hopkinson pressure bar(SHPB) test system,and a coal and rock dynamic mechanics constitutive model was discussed. The results show that there is a posi- tive correlation between the loading strain rate and the impact velocity of coal / rock samples,and the coal / rock samples have obvious phased mechanical response characteristics under different impact velocities. According to the difference of impact velocity,the stress-strain curves are divided into three types of compact deformation,plastic deformation, plastic softening deformation or plastic hardening deformation. In addition, the fracture characteristics of coal / rock samples all have significant strain rate dependence. Under low impact velocity,the samples are in the form of brittle failure. With the increase of impact velocity,the ductile fracture characteristics of samples gradually appear. Besides, based on the stress-strain constitutive relations and dynamic failure characteristics of coal / rock samples,a viscoelastic damage constitutive model with low-medium-high strain rate response was proposed. The results of numerical simula- tion show that the simulated curve has a good fitting accuracy with the measured curve,which proves the validity and rationality of the viscoelastic damage constitutive model with low-medium-high strain rate response.