Abstract: The mechanical properties of solid propellants are of great significance to the structural integrity and operational reliability of solid rocket motors. To tackle the typical issue of reduced maximum elongation in high-strength HTPB composite propellants, a diol-type chain extender LAP was introduced as a reinforcing agent. The effects of reinforcing agent content, curing parameters, and bonding agent on the uniaxial tensile properties of the propellants were systematically investigated, and their mechanical behavior under different temperatures was also analyzed. Meanwhile, small-angle X-ray scattering (SAXS) was used to characterize the microphase-separated structure of the binder matrix. The results show that the introduction of LAP shifts the SAXS scattering peak of the binder matrix from approximately 11.7 nm to 6.6 nm, with a clearer peak profile, indicating an enhanced microphase-separated structure. This structure serves as physical cross-linking points, which significantly improve the tensile strength of the propellant, reaching 3.36 MPa at 20 ℃, while the elongation at break remains at 27%.