To explore the tensile properties of glass fiber reinforced polymer (GFRP) bars exposed to different aggressive environments, the tensile properties of GFRP bars before and after conditioned were tested. Effects of environmental categories and exposure periods on the tensile properties of GFRP bars were analyzed. The GFRP bars were conditioned in two different ways: the first part of GFRP bars were directly immersed in different solutions (alkaline solution, salt solution, and tap water) with the immersion period of 180 days; and the second part of GFRP bars were embedded in sustained-loaded concrete beams exposed to outdoor air and NaCl solution in drying-wetting cycles with the total period of 366 days. Scanning electron microscopy (SEM) was used to analyze the microstructure changes of the GFRP bars placed in concrete beams. Test and analytical results show that the tensile properties of the GFRP bars immersed in aggressive solutions had different degrees of degradation with increasing exposure period. After 180 days of exposure to solutions, the tensile strength decreased by 30.0%, 21.3%, and 11.3% in alkaline solution, salt solution, and tap water, respectively. For the GFRP bars embedded in concrete, the average loss of the tensile strength of the bars was about 10% after 366 days. Based on the test results and SEM microstructure analysis, it can be deduced that the main reason of the degradation of conditioned GFRP bars is the debonding between the fibers and the resin matrix. Finally, the tensile strength degradation of GFRP bars under two conditions were fitted based on Arrhenius model. In addition, the long-term performance predictions of GFRP bars exposed to alkaline solution and salt solution and embedded in concrete beams were proposed, respectively.