Abstract: Bending test can be used to assess the quality of welded joints, however, there are great differences in the different standards about how to select the proper bending diameter for the bending test. In this paper, firstly, the theoretical formula for bending elongation of the welded joint under ideal conditions is deduced, and the finite element method is used to analyze mechanical properties of the welded joint, and the correctness of the theoretical formula is verified. Further, the theoretical formula is modified according to the analysis results, and the formula for minimum bending diameter under the two bending methods is established respectively. Then the new formula is verified by carrying real bending test on welded joints of different specifications. Finally, calculation result of the formula for minimum bending diameter are compared with the standard of Chinese Classification Society. The results showed that the bending diameter recommended by the Chinese Classification Society is closer to the calculation result of three point bending, but when the bending diameter is small, it is more recommended to use the roller bending method.