Water security is the premise for island sustainable development. Rapid urbanization and rising tourism industry have reshaped the water system in China's island cities, and it is necessary to reveal the characteristics of the aforementioned impacts in relation to strength and duration time to serve water management. Here, we presented a framing to interpret the nexus between island developments on water security. Subsequently, their coevolutionary trend and mutual impact effects (coupling coordination degree and response period) were measured by mathematical models, respectively. Results demonstrated that the equilibrium of the water system has shifted from nature- to human-dominated since 2010. Interestingly, the coupling coordination degree between water security and island development showed an upward increasing trend, across the study periods. Moreover, water security exhibited positive and negative shock responses to tourism and urbanization, over 1- and 7-year response periods, respectively. Overall, the findings from this case study provide a quantitative paradigm for island sustainable management, and are expected to inform local decision makers on warning signals of sustainability loss, at a temporal scale.