The performance degradation rule and mechanism of performance of glass fiber reinforced polymer（GFRP） bars in simulated concrete pore solution and real concrete environment were explored. The GFRP bars were conditioned in two different ways. The first part of GFRP bars were immersed in alkali solution（AS） and the second part of GFRP bars were wrapped with concrete and then submerged in tap water（CS）. The mechanical property degradation of GFRP bars was analyzed by using short beam shear method. Microstructure and glass transition temperature（T_g） of GFRP bars after aging in two environments for 183 days were analyzed using scanning electron microscopy and differential scanning calorimeter respectively. The results demonstrate that the degradation rate of GFRP bars increases with the increase of temperature. After 183 days aging treatment at 60 ℃ in AS environment， the interlaminar shear strength of GFRP bars is retained at 48.6%， T_g is reduced by 9.2%， the separation between fiber and resin is observed and a few holes may be seen in the resin. However， after the same treatment of GFRP bars in the CS environment， the interlaminar shear strength of GFRP bars is retained at 61.4%， T_g is reduced by 3.4%， and the fibers has shallow pits caused by erosion. Finally， the durability prediction model of GFRP bars serving in Beijing is established by using Arrhenius equation.