The failure behavior of glass fiber reinforced polymer (GFRP) in high temperature and high humidity environments, i.e. 60 ^oC-80%RH, 60°C-95%RH, and 80 ^oC-95%RH was investigated by tensile, compressive, flexural, and impact mechanical property tests, combined with morphological characterization, infrared spectral analysis, and moisture absorption rate test. The results show that in high temperature and high humidity environments, humidity mainly affects the saturated water absorption of GFRP, and temperature mainly contributes to the increasing water diffusion coefficient, which leads to the most serious damage to GFRP at 80 ^oC-95% RH. The aging degree of GFRP shows no obvious difference in the other two environments of 60 ^oC-80%RH and 60 ^oC-95%RH. Among the results of tensile, bending, compression, and impact performance test, those of the compression test reveals that the most significant decline in compressive strength of GFRP immerged at 60 ^oC-80%RH and 60 ^oC-95%RH, decreasing by 18.18% and 22.22% respectively after 49 d of aging. This may be attributed to the dissolution and plasticization of the resin matrix after water absorption by GFRP; Those of the impact strength test shows that the most significant decrease in impact strength of GFRP occurred at 80 ^oC-95% RH, decreasing by 51.43% after 49 d of aging, which is mainly due to the destruction of the resin/fiber interface.