Abstract: The early-stage service performance evolution of glass fiber reinforced polymer (GFRP) bars in tropical marine environments remains insufficiently understood. In-situ exposure tests (atmospheric, tidal, and seawater immersion) and accelerated aging tests (ultraviolet radiation, salt spray exposure, UV+salt spray coupling, and UV+condensation cycling) were conducted in a comparative manner. Macroscopic mechanical testing combined with microstructural characterization techniques was employed to reveal the degradation behavior and deterioration mechanisms of the fiber-resin interfacial bonding under different environmental conditions, and to establish equivalence relationships between accelerated aging and in-situ exposure. The results indicate that the atmospheric environment is mainly dominated by ultraviolet radiation and condensation effects, while the tidal environment experiences enhanced effective ultraviolet irradiation due to reflection and scattering from water surfaces. In contrast, the immersion environment is primarily controlled by moisture and salt penetration. FTIR and SEM analyses further confirm that, under the combined effects of post-curing, hydrolysis, and molecular chain scission, the fiber–resin interfacial structure gradually evolves from a dense morphology to a roughened and partially debonded state. Based on the identified degradation mechanisms, three equivalence relationships—UV+condensation–atmospheric, UV–tidal, and salt spray–immersion—were established, with acceleration factors of 1.7, 1.5, and 1.3, respectively. The corresponding MAPE values were 2.25%, 2.90%, and 1.59%, demonstrating the reliability of the proposed accelerated aging approach. These findings provide a theoretical basis for durability evaluation and accelerated test design of GFRP bars in tropical marine environments.