The viscoelastic aging behavior of composite solid propellant grains is studied by a three-dimensional thermoviscoelastic constitutive model with aging evolution, in which the crosslinking degree is taken as an aging variable. The resulting viscoelastic constitutive equations are numerically discretized into incremental form by integration algorithms, and the stress-updating method is presented according to incremental law. Subsequently, the Abaqus-based user material subroutine and its structural analysis module are constructed and validated through typical examples. Studies show that the results of structural analysis have high accuracy and are in well agreement with each other. The designed analysis module presents an effective aging-mechanical analysis platform for further lifespan estimation of solid rocket motors.