Exploring and demonstrating the justice of the Chinese navy defending underwater safety not only contributes to revising and developing the theoretical system of classic justice war theory, but also provides the Chinese military activities with a basis of legitimacy in international opinion. Based on a case study of the incident that ‘the Chinese naval identifies and verifies the unmanned submersible of the US military’ at the end of 2016, the paper analyzes the US military’s use of military submersibles to carry out military detections in the Chinese coastal areas which goes against the theoretical interpretation of ‘aggression’, ‘self-defense’, and ‘pre-defense’ in the classic justice war theory. The purpose of the Chinese Navy’s operations in the South China Sea is to prevent underwater unidentified devices from jeopardizing the navigation and personnel safety of passing ships. It by no means indicates the expansion of the territorial sea, and it in no way interferes the freedom of navigation in relevant waters of the South China Sea. Such a practice of the Chinese Navy is justified in exercising the right of pre⁃defense, and the entire identification process follows the principle of distinction and appropriateness, which conforms to stipulations of the classic military ethics theory on the justice of military operations.