Abstract: Aiming at the problem of weld cracks in tie rod seat of HXD2 electric locomotive running on a line crossing Qinling Mountains, the method of combining test and simulation was used to study the problem. Fatigue striations were found on the surface of cracks, which indicated that fatigue failure occurred in the fillet weld of tie rod seat during service, and cracks initiated at the root of weld and were characterized by multiple sources. After initiation, cracks expanded from the center to both sides and from the root to the surface of weld, and the angle between crack propagation path and horizontal direction was approximately 45° on the cross section of welded joint. Statistical analysis and simulation showed that occurrence of fillet weld crack could be related to gap value between tie rod seat and substrate, and the larger the root gap was, the worse the fatigue resistance of fillet weld was. Analysis showed that main mechanism might be that when the gap was small, the bottom surface of tie rod seat was more likely to contact with the surface of frame substrate, and the substrate surface shared part of the load after contact. Therefore, the stress amplitude of weld was small when the gap was small, but when the gap was large, not only the stress amplitude was large, but also the wedge-shaped gap end was easy to appear at weld root.Highlights: Fatigue failure behavior of fillet weld of tie rod seat of HXD2 electric locomotive crossing Qinling Mountains was analyzed, the correlation between fatigue failure of frame fillet weld and root gap size of fillet weld was revealed, and mechanism of root clearance affecting fatigue resistance of tie rod seat fillet weld was clarified.