Abstract: The varying levels of stress concentration resulting from different welding details in welding structures are the primary causes of structural fatigue failure. To assess the fatigue performance of structures with welding details, obtaining fatigue life (S-N) curves for materials with various welding details is essential. The influence mechanism of geometric dimensions and material properties of welding details on the fatigue life of welding materials was analyzed. The influence of welding details on the fatigue life of the welding material was quantified by determining the stress concentration factor and the stress change gradient. By analyzing the stress change gradients of different welding details with the same stress concentration factor, a correction factor for stress concentration was calculated from the difference in stress gradient between micro-void defects and other welding details. Different welding details were equivalently transformed into micro-void defects corresponding to the modified stress concentration factor, and then the S-N curve of all welding details was equivalently determined through the S-N curve of the welding material with micro-void defects. Finally, simulated slag inclusions in welding details of high-temperature ceramics for producing steel pipe weld specimens were used for uniaxial high-frequency fatigue tests, verifying the method’s feasibility to determine the S-N curve of welding materials using welding details equivalent to micro-void defects.