Failure assessment diagram（ FAD） is usually used to evaluate the safety of welded structures containing cracks.There are three assessment levels in BS7910. As the assessment level increases,accuracy is improved and more data is required.This standard is widely used in safety assessment of pressure vessels and pipelines. The working level of offshore crane is relatively high and fatigue cracks can be found on it during the routine inspection. However,due to its high cost and long cycle of maintenance,the productivity is seriously affected. Therefore it is vital to commence safety evaluation on the cracked structure. This paper utilized Shao Yongbo’s experimental data to establish the 3C curves of the cracked structure of offshore crane,and compared it with the 2A curves of the BS7910 standard. The result shows that the tendencies of both curves are the same and 2A curve is more conservative. Thus the structure can be evaluated according to BS7910. This paper offers the critical load of T-plate that contains certain dimension of crack according to level 2A evaluation.