Abstract: Orthotropic steel bridge decks are widely used in large and medium span bridges due to their light weight and high strength. Currently, the steel bridge decks mainly adopt welding as the connecting method. The discontinuous geometry and inevitable welding defects from welding often lead to stress concentration, making fatigue cracks likely happen at these joints. This study analyzes the fatigue performance and crack propagation at rib-to-deck connections, considering the impact of initial defects. The research results show that the undercut defect has a great influence on the structural fatigue performance, resulting in a decrease of about 28% in fatigue strength. The partial penetration defect has little effect on the structural fatigue performance, resulting in a decrease of about 6% in fatigue strength. For specimens with undercuts, the S-N curve can be adjusted by subtracting 4 standard deviations from the curve without defects. Fatigue strength can be divided by a safety factor of 1.4. According to the Paris formula, the relevant crack propagation parameters are calculated, and the POWER criterion parameters suitable for members with and without undercut defects are obtained. The fatigue performance and crack propagation of the rib-to-deck and rib-to-diaphragm welded details with and without undercut defects are analyzed by using the extended finite element method. The finite element simulation results are consistent with the experimental results and, with the crack morphology of the actual bridge. It is shown that the crack propagation model can effectively evaluate the fatigue life, crack development stage and shape of fatigue detail. The results of study offer a foundation for exploring residual fatigue life and crack propagation at welded joints with initial defects, contributing valuable insights into crack propagation mechanism, repair measures and repair time.