Abstract: Hydrogen-induced fracture is an important failure mechanism of metallic materials in hydrogen environments. At present, the degradation behavior of the mechanical properties of girth weld materials in high-grade pipelines under hydrogen environments remains unclear, and their fracture behavior cannot yet be accurately characterized. To address this issue, this study established the relationship between dislocation trap density and plastic strain, and proposed calculation formulas for hydrogen trap densities at carbide interfaces and martensitic grain boundaries. Based on Oriani’s equilibrium theory, the relationship between the occupancy of trap sites and that of lattice sites was derived, and the interaction mechanism among lattice hydrogen concentration, trapped hydrogen concentration, and trap binding energy was clarified. On this basis, a hydrogen diffusion constitutive equation was derived from the hydrogen diffusion governing equation and the mass balance equation by considering the coupled effects of hydrogen concentration gradient, hydrostatic stress gradient, and plastic strain gradient. Furthermore, by introducing a hydrogen damage factor into the processes of micro-void nucleation, growth, and coalescence, a fracture-damage constitutive model for high-grade pipeline girth welds under hydrogen environments was established on the basis of the Complete Gurson Model, considering the coupled effects of hydrogen media and complex stress states. The constitutive model was implemented through the fully coupled logic of multiple user subroutines, including UMATHT, UMAT, and UEXTERNALDB, and its accuracy was validated experimentally. The results show that the relative errors between the simulated and experimental load-displacement curves of the base metal are approximately 1.25% and 1.45% in air and in a 20% hydrogen-blended environment, respectively. For heterogeneous girth weld materials, the error between the simulated R-resistance curve and the experimental results is less than 10%. A fracture-damage constitutive model for high-grade pipeline girth welds under hydrogen environments was established in this study, which provides technical support for investigating the fracture behavior of girth welds in hydrogen-blended natural gas pipelines and for developing fracture assessment methods for girth welds under different complex service conditions.