To address the significant reduction in service life of concrete structures caused by harsh natural conditions in the South China Sea island and reef region, this study systematically investigates the performance degradation patterns and service life of concrete in typical island and reef environments (underwater, tidal, and salt-spray environments). The results show that, for concrete in different island and reef environments, the time-varying trends of performance indicators show an initial increase followed by a decrease, and its time-varying model is formulated as a power-exponential function. Building upon this foundation and considering size effects, a time-varying model of concrete performance degradation accounting for multifactorial coupling effects is developed. This subsequently enables the establishment of a service life prediction model specifically adapted to the South China Sea island and reef region, through which the evolutionary patterns of concrete lifespan are thoroughly elucidated. Concrete service life exhibits a positive correlation with both compressive strength and concrete size, larger dimension or higher compressive strength leads to progressively extended durability. And, significant variations in service life are observed across different island and reef environments, with the following durability ranking (in descending order): underwater, salt-spray, and tidal environments. Finally, based on the established power-function model between the service life and concrete sizes, and the critical size values for concrete without additional protective measures is derived.