Abstract: The refueling pod adaptor of an aircraft was found to have crack and leak during testing. The refueling pod adaptor was welding assembly. The crack was found near R angle of the cover plate, whose material is ZL116 alumium alloy. The failure mode and cause were analyzed by macro and micro observation, microstructure examination, chemical composition analysis, and mechanical property testing. The results show that the crack of the adapter was caused by the co-effect of the poor plasticity and excessive stress. The excessive stress mainly came from the structure stress concentration, welding stress, shape correction stress, assembly and test stress. It is suggested that the cover plate should be replaced with a pre-stretching plate or the distance between R angle and the welding path should be increased.