Abstract: To achieve integrated simulation of solid rocket motor grain responses under sequential storage-ignition loading conditions and investigate the damage evolution of propellant materials under large deformation, this study takes HTPB propellant as the research subject. Constant-rate tensile failure tests and stress relaxation experiments were conducted to investigate the rate-dependent mechanical properties and damage evolution mechanisms of HTPB propellant. Based on the generalized Maxwell model and the Zhu-Wang-Tang constitutive model, a nonlinear viscoelastic constitutive model was developed to simultaneously characterize long-time-scale relaxation and creep behaviors and short-time-scale ignition pressurization responses. A strain-rate-dependent damage factor was introduced to quantitatively describe the softening behavior of HTPB propellant induced by large-deformation damage. The results demonstrate that, within a strain range of 30%, the proposed model enables continuous simulation of solid rocket motor grain responses from long-term storage to ignition pressurization and reasonably captures the damage-induced softening behavior of HTPB propellant. The proposed model overcomes the limitation of conventional approaches, which cannot continuously simulate storage and ignition conditions, and provides a new continuous simulation method for the structural integrity assessment of solid rocket motors throughout their entire life cycle.