Abstract: Ultra narrow gap laser welding(ultra-narrow gap laser welding, Ultra-NGLW)is an advanced welding technology for thick bainitic steel, which has the advantages of low heat input, low stress and deformation of welded joints.By conducting high-frequency tensile fatigue tests on Ultra-NGLW joints with different "A-B" ratios, S-N curves of 10 mm thick Ultra-NGLW joints with different wire filling depths were plotted, and by comparing and analyzing the S-N zone lines and fatigue fracture characteristics of the joint, the main factors affecting the fatigue performance of Ultra-NGLW joints were identified. The experimental results indicate that as the depth of laser wire filling increases, the fatigue performance of the joint gradually decreases. When the filling depth is 0 mm, the fatigue strength of the joint is 337.5 MPa, while the filling depth reaches 5 mm, the fatigue strength tends to stabilize at around 270 MPa. The fatigue source of Ultra-NGLW joints is mainly welding defects such as pores and inclusions in the coarse-grained and fusion zones of the weld. Under the action of high-frequency tensile fatigue cyclic loads, more cracks are formed in the martensitic brittle hard structure in the coarse-grained zone, causing the fatigue source cracks to rapidly expand and connect the cracks in the brittle hard structure until the joint fails.