Founded on the energy storing characteristics of microstructure during irreversible deformation, a viscoplastic constitutive model with no yielding surface introduced was developed for single crystals by adopting a spring-dashpot mechanical system. Both plastic dashpots reflecting the material time-independent responses and Newtonian dashpots mirroring the material time-dependent viscous responses were introduced to describe the viscoplasticity of slip systems. The single crystal constitutive model was established based on the thermodynamics of internal variables and the theory of absolute reaction rate. By implementing the KBW self-consistent theory, a polycrystal viscoplastic constitutive model was formed. The numerical analysis in corresponding algorithm was significantly simplified as no searching process for the activation of the slip systems and slip directions was required. The numerical simulation of creep-plasticity behaviors demonstrated excellent agreement with the corresponding experimental data.