Abstract: The main S-N curve method is a new method to calculate the fatigue life of welded structures. It is widely used in the fatigue analysis of welded structures. The equivalent structural stress is taken as the key parameter to calculate the fatigue life of structures with different loading modes and welding forms by single one S-N curve. Based on the principle of the master S-N curve method, a database of welded structure was created through different materials and welding forms of fatigue test. The effects of correction parameters such as thickness, bending ratio and multi-axial stress were taken into concern to study the different master S-N curve equations fitting method and its standard deviation base on the principle of least squares method. The special software for the master S-N curve fitting method was developed. Based on VC++ programming environment the main calculation functions of nominal stress, structural stress, equivalent structural stress, shear structural stress, multi-axial structural stress and initial crack correction were compiled. The multi parameter comparative analysis of the test data was completed in this software. It provides a technical basis for the study of fatigue life assessment and influencing factors analysis of welded structures.