Abstract: A phenomenological model, including nonlinear elasticity (equation of state), rate-dependent plasticity, and void nucleation and growth, was proposed to simulate deformations of U-6%Nb alloy under the condition of high strain rate. The diagonally implicit Runge-Kutta (DIRK) method was used to solve the constitutive rate equations, and stability and accuracy of the solution to thermoviscoplastic constitutive relations were improved. The proposed model was used to simulate the plate impact experiments on U-6%Nb alloy by Tonks D L, et al. Calculated results by the proposed model are in better agreement with the experimental data than the simulated results by Addessio F L, et al.