Abstract: High lock nuts were made from 7075-T73 aluminum alloy that was obtained by T73 heat-treating 7075-T6 aluminum alloy. After serving in an atmospheric environment for about 3.5 a, many nuts cracked. The crack reason was analyzed by chemical composition analysis, microstructure and fracture morphology observation, residual stress testing and other methods. The results show that stress corrosion cracking occurred in the 7075 aluminum alloy high lock nut during service. After the high lock nut was installed, the circumferential residual tensile stress was generated at the closing part, and the surface oxide film was cracked, making the metal substrate in contact with corrosive media in atmosphere. The second phase precipitated locally and continuously along grain boundaries during the heat treatment from T6 to T73 states, providing channels for crack initiation and propagation. Under the combination of these factors, stress corrosion cracking occurred in the high lock nut.