Abstract: The early-stage service performance evolution of glass fiber reinforced polymer (GFRP) bars in tropical marine environments remains insufficiently understood. To address this, a comparative study was conducted using in-situ exposure tests (atmospheric, tidal, and seawater immersion) alongside accelerated aging tests (ultraviolet radiation, salt spray exposure, UV+salt spray coupling, and UV+condensation cycling). Macroscopic mechanical testing combined with microstructural characterization was employed to reveal the degradation behavior and deterioration mechanisms of the fiber-resin interfacial bonding under various environmental conditions. The results indicate that in environments involving UV radiation, the interlaminar shear strength (ILSS) of GFRP bars initially increase and then decrease. The UV+salt spray coupled environment has the most pronounced effect on ILSS, leading to an 18.19% reduction after 40 days of exposure. Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscope (SEM) analyses verify that, due to the synergistic effects of post-curing, hydrolysis, and molecular chain scission of the resin matrix, the fiber-resin interface gradually transitions from a dense state to a rough structure accompanied by debonding.