The objective of this study is to investigate defect tolerance of HFMI-treated welds in bridge application for life extension based on numerical FE simulations. Following the authors’ previous study on 3D HFMI simulation to the rat-hole welds, 3D crack propagation analysis (CPA) based on linear elastic fracture mechanics (LEFM) was performed, using the HFMI-treated rat-hole welds including different defect depths in the HFMI-treated welds. At first, compressive residual stress introduced by the 3D HFMI simulation was considered over these crack faces in the HFMI-treated welds. Crack opening and closing stress at the crack tip in the HFMI-treated weld were investigated considering the crack opening-closing behavior. The results indicate that cracks in the HFMI-treated welds would propagate after the HFMI-treated crack surface opened. Then, 3D CPA based on LEFM was carried out under fatigue load. The results show that it is difficult to represent crack propagation behavior in a deeper region where the given residual stress distributions transition from compressive to tensile residual stress. The calculated fatigue life results demonstrated that the life extension may be obtained effectively when HFMI treatment is performed to pre-fatigued welds including fatigue cracks that are shallower than 1.5 mm in depth.