Abstract: Based on the ladder method and the group method, the S-N curve of stainless steel welding fittings was drawn. According to comparing drawn S-N curve with the ladder method data, the turning point of S-N curve requires turning transition. In order to make the S-N curve and the breaking point N₀ have more engineering application value, the theory and method of transition point connected with variable slope curve was proposed. The feasibility of using variable slope method has been verified by experiment, and the two curves were compared by analysis. The results show that the new method of drawing the S-N curve can display the performance of welding joints spot more accurately.