Passively confined dynamic impact experiments on PBX1314 specimens are performed by employing aluminum jackets with split Hopkinson pressure bar. The axial and radial stress history curves are measured in the experiments, and the characterizations of the behavior of PBX1314 under dynamic multi-axial loads are studied. A constitutive relation is developed for modeling the dynamic mechanical responses of PBX1314 by using the Boltzmann superposition principle with a Prony series representation. The material parameters of PBX1314 can be obtained by fitting the modulus master curve. Detailed finite element simulations of the passively confined tests are carried out to evaluate the measure accuracy of the device to the material mechanical behavior. The correctness of the constitutive relation is verified by comparison the finite element simulations with the experiments. Good agreements are obtained.