Abstract: To establish a method for failure assessment and critical flaw evaluation of X60 hydrogen transmission pipeline girth welds under different hydrogen pressures, all-weld-metal tensile tests, fracture tests, and fracture-surface analyses were conducted to investigate the mechanical response and fracture behavior of the welded joints in 10 MPa N₂, 1 MPa H₂, and 10 MPa H₂ environments. Based on the J–R resistance curves and elastic–plastic fracture parameters obtained from the fracture tests, failure assessment curves were constructed using BS 7910 Option 3, and the relationships between the critical fracture toughness K_IC and the critical flaw dimensions a and 2c were analyzed. The results indicate that increasing hydrogen pressure has only a limited effect on the strength of the girth welds but significantly reduces their plastic deformation capacity and crack growth resistance. The fracture mode gradually changes from dimple-dominated fracture at low hydrogen pressure to quasi-cleavage-dominated brittle fracture at high hydrogen pressure. Correspondingly, the failure assessment curves contract toward the failure region, and the flaw tolerance of the girth welds decreases markedly under high hydrogen pressure. The results provide a theoretical basis for determining flaw acceptance criteria and establishing integrity assessment methods for hydrogen transmission pipeline girth welds.