Pultruded glass fiber reinforced polymer (GFRP) composites are extensively applied in the field of ultra-high voltage power transmission due to their excellent electrical and mechanical properties, and the design considering its performance degradation characteristics under fatigue loading is the key to the application. This study carried out static load tests and fatigue tests under different conditions with the pultruded unidirectional GFRP. A fatigue life prediction model was established based on a piecewise linear constant life diagram determined by S-N curves. According to the stiffness degradation law under different stress ratios, a modified damage accumulation model based on the improved trigonometric function was presented to describe the nonlinear stiffness degradation process under both tension-tension and tension-compression fatigue loads. The correlation model between residual strength and residual stiffness was established to predict the strength degradation precisely. The results show that the stiffness degradation processes of GFRP under tension-tension and tension-compression fatigue loads are significantly different. The model proposed in the paper can accurately predict the residual strength under the two conditions. Under compression-compression fatigue load, there are two different patterns for the stiffness degradation processes, and the residual strength test shows that the strength within the first 70% fatigue life does not exhibit a significant degradation. The prediction model for the residual strength and residual stiffness of GFRP under different fatigue loading provides a guide for the durability design of GFRP.