Abstract: When an underwater vehicle hanging to a helicopter conducted an air-drop test, the inactive wire of the water activated battery linked to the hanger was not pulled out but fractured, resulting in the failure of the water activated battery. In order to find out the failure cause, macroscopic and microcosmic analysis, metallographic structure examination, energy spectrum analysis and hardness testing were conducted. The results show that the fracture was due to the surface cracks of the inactive wire after bending. In order to prevent such failures, it is proposed that the technical requirements and inspection process of the steel wires should be improved to prevent surface cracks. The research is of actual significance to the reliability design of the inactive wires of water-activated battery, and provides necessary basis for perfecting relevant technologies of factories, armies and so on.