Abstract: The girth butt weld attaching to the cast steel joint and the hot-rolled tube are the important object of the fatigue study of the cast steel joint due to the material nonlinearity and geometric nonlinearity across the weld． However，the research on this issue rarely saw in China and the nominal stress method was not applicable to it． In the paper，the hot spot stress method was used while the results of hot spot stress extrapolations were compared． To reduce the impact of the stress concentration，different weld details were designed． The hot spot stress curve was drawn under consideration of the tube’s thickness and the other two parameters．