Abstract: To obtain the long-term development length equation of the glass fiber-reinforced polymer (GFRP) bar under the coupling effect of seawater immersion and sustained load, a short-term development length equation was proposed first according to the collected 81 GFRP bar-reinforced concrete beam with pullout failure. Then, the bond strength of the pullout specimen under the coupling effect of seawater immersion and sustained load was tested, and the bond strength reduction factor was obtained with a prediction theory. Based on the bar’s bond strength reduction factor and the tensile strength reduction factor deduced from others’ tests, the short-term development length equation was modified. Finally, the long-term development length equation of the GFRP bar for the beam with pullout failure was established. The results show that the change in the long-term development length of the GFRP bar is mainly caused by the reductions in bond strength and tensile strength. After the coupling effect of seawater immersion and sustained load for 50 years, when the annual average temperatures of the environment are 8℃, 13℃, 18℃, 23℃ and 28℃, the bar’s bond strength retentions are 0.60, 0.60, 0.56, 0.56 and 0.52, respectively. The corresponding tensile strength retentions of the GFRP bar are 0.63, 0.56, 0.49, 0.42 and 0.35, respectively.