Abstract: In order to establish the theoretical model for long-term stiffness of fiber reinforced plastic pipes, and predict the long-term stiffness of them, using the autonomous designed constant displacement loading test apparatus, the testing investigations for long-term stiffness of glass fiber reinforced plastic pipes with of pure hoop winding and pure cross winding plies were performed under constant displacement conditions of different initial deflections firstly. Then, stiffness vs time broken line log-log regression model of glass fiber reinforced plastic pipes under different initial deflections, linear regression model of stiffness degradation rate after 50 years vs initial deflection were established based on the test, thus quadratic surfaces prediction model of glass fiber reinforced plastic pipes stiffness degradation rate vs time and initial deflection was proposed. Finally, the residual stiffness of glass fiber reinforced plastic pipes after 50 years was predicted, and the effects of time and initial deflection on stiffness of glass fiber reinforced plastic pipes were investigated. The results show that the ability to resist stiffness degradation of glass fiber reinforced plastic pipes with pure hoop winding ply are much more superior to that of glass fiber reinforced plastic pipes with pure cross winding ply, pure hoop winding ply can improve the stiffness of glass fiber reinforced plastic pipes and the ability to resist radial deformation of them effectively, glass fiber reinforced plastic pipes with pure hoop winding ply have better long-term mechanical performances. The test data when the time is 8 313.2 h indicates that the proposed prediction model for stiffness degradation rate of glass fiber reinforced plastic pipes has relatively high precision and relatively great applicability.