Deformation and fracture of a hydroxyl-terminated polybutadiene (HTPB)/ammonium perchlorate (AP)/aluminum solid propellant under quasi-static tensile loading are investigated by in situ synchrotron X-ray micro computed tomography (CT) and CT-image-based finite element method (FEM) modeling. Bulk stress–strain curve of the solid propellant, and the evolution of particle morphology, and mesoscale strain and particle displacement fields are obtained. Based on tracking and statistics, an automated analytical method is proposed to analyze the relationship between microcrack nucleation and initial structure. The AP particles undergo negligible deformation and orientation changes during tensile loading. Microcracks are mainly nucleated via tension-induced debonding at the maximum surface curvature of the AP particles, and propagate along the curvature gradient around AP particles. Larger AP particles are more prone to debond, and Al particles play a negligible role in deformation and fracture. Display omitted