ABSTRACTChanges in the aging properties of hydroxyl‐terminated polybutadiene (HTPB)‐based propellants are typically observed as variations in mechanical properties at the macroscopic level. This study establishes an indentation contact model based on indentation theory and viscoelastic constitutive equations to examine how material parameters evolve during aging and their implications for service performance. Experimental analyses, including uniaxial tensile tests on dumbbell specimens, tensile tests on rectangular specimens, and gel fraction tests, were conducted on HTPB‐based propellants with various aging durations. Mathematical models were developed to characterize the changes in elongation, gel fraction, and initial modulus during storage. Results indicate that increased aging time reduces maximum elongation, while the gel fraction and initial modulus gradually increase. The initial modulus within the effective service range is determined to be between 3.79 and 7.71 MPa. These findings provide theoretical support for enhancing the mechanical property assessment of HTPB‐based propellants.