A reasonable constitutive model is the prerequisite to accurately estimate the mechanical behaviours of the composite propellant and the structural analysis of solid rocket motor grain. Through coaxial tensile tests, the viscoelastic properties of the solid propellant, especially dewetting, under actual loading is studied. The concept of dewetting point is proposed to describe the behaviour of dewetting. Based on the experimental results, a nonlinear viscoelastic constitutive model considering strain rate was applied to composite propellant and modified by the dewetting factor to extend the original one to a wide strain variation. Based on the updated Lagrangian approach, the incremented form of the constitutive model is deduced using the updated Kirchhoff stress tensors and strain tensors. By the numerical method and user defined material subroutine, the model is implemented to the 3D structural analysis of the grain during the ignition pressurization procedure. The model is validated by one set of experiments and further grain structural integrity analysis is carried out to study the influence of the pressurization rate and dewetting elongation.