ABSTRACTThis study conducted systematic in‐situ tensile scanning tests on hydroxyl‐terminated polybutadiene (HTPB) propellant under biaxial tensile conditions. It successfully acquired in‐situ experimental data at different tensile ratios. Through in‐depth analysis of the experimental data, the study elucidated the damage evolution mechanism of HTPB propellant specimens during the biaxial tensile process. The research utilized advanced imaging techniques to visually analyze the mesoscopic damage evolution process of two‐dimensional slices of the solid fuel, with porosity being used as a key indicator of damage variables for the quantitative description of the damage process under biaxial tensile. The study combined statistical principles with damage theory to derive a damage evolution equation describing the change of porosity with strain. A detailed comparative analysis between the theoretical model and experimental results validated the correctness and feasibility of the damage evolution equation, providing a new theoretical basis and method for damage assessment of solid propellants under complex stress states. This study enriches the understanding of the mechanical behavior of HTPB propellants and offers significant experimental data and theoretical models for research in related fields.