<p>This article focuses on underground engineering of islands, specifically investigating the minimum safe thickness (MST) of overburden rock and the safety of the surrounding rock. Initially, an improved ‘three-zone’ mechanical model was established to derive the MST expressions for various overburden structures. Subsequently, the reliability of this model was verified through a real-world engineering application. Using the Finite Discrete Element Method (FDEM), the safety of surrounding rock in different overburden structures was verified by analyzing the displacement, stress and seepage fields. The simulation results indicate that the SCL structure can ensure superior safety of surrounding rock and should be prioritized in the engineering site selection. Furthermore, based on the Japanese Minimum Seawater Waterflow Volume (JMSWV) method, we proposed a seepage instability criterion for assessing the safety of surrounding rock in the development of underground space on oceanic islands. Ultimately, the differences in calculating the MST of the water-resistant between the proposed theory and methods for terrestrial karst tunnels were explained. The findings confirm that the proposed method can effectively carry out underground space development and evaluate the safety of surrounding rock on oceanic islands.</p>