Abstract: The stress intensity factor (SIF) serves as a critical index to evaluate the structural integrity of cracked solid rocket motor propellant grain, particularly under ignition and pressurization loading. However, existing measurement methods for stress intensity factor of solid propellants primarily depend on indirect inference from macroscopic load data. Such methods are strongly affected by off-crack-tip interference and prone to systematic measurement errors. The high-precision two-dimensional digital image correlation（2D-DIC） method was adopted to capture the surface displacement field of the propellant specimens. Based on the analytical relationship between the crack-tip displacement field and the SIF, the crack-tip SIF of the propellant was calculated via the displacement extrapolation method. Furthermore, an effective calculation sector screening strategy was established, which takes the Mode Ⅱ stress intensity factor and the determination coefficient of linear fitting as dual criteria, based on which the fracture toughness of the HTPB propellant was evaluated. The results show that under loading, the propellant crack tip presents evident nonlinear deformation and localized micro-damage, resulting in a strong angular dependence of the extrapolated stress intensity factors. With the proposed screening strategy, an effective computational domain consistent with the crack-tip asymptotic theoretical solution was accurately determined. Within this optimized computational domain, the evaluated Mode Ⅰ stress intensity factors exhibit favorable repeatability and convergence stability. The critical fracture toughness of the tested HTPB propellant was experimentally measured to be 1.22 \mathrmM\mathrmP\mathrma\cdot\mathrmmm^1/2 .