In response to the significant deterioration of the load-bearing capacity and ductility of reinforced concrete (RC) beams caused by steel bar corrosion induced by chloride ion erosion in marine environments, as well as the problems of existing reinforcement technologies such as proneness to corrosion and poor interfacial bonding, this paper proposes a technical method for strengthening corroded RC beams with basalt fiber grid-reinforced geopolymer composite (FRGC). Fifteen RC test beams with different longitudinal reinforcement ratios were designed and fabricated. Among them, 6 beams were non-corroded, including 3 unstrengthened beams and 3 beams strengthened with single-layer FRGC; 9 beams were subjected to sustained loading accelerated corrosion, with a theoretical corrosion rate of 13% for the tensile longitudinal reinforcement, including 3 unstrengthened beams, 3 beams strengthened with single-layer FRGC, and 3 beams strengthened with double-layer FRGC. Through four-point bending static tests, this study investigated the influences of longitudinal reinforcement corrosion rate, number of FRGC strengthening layers, and longitudinal reinforcement ratio on the flexural performance of the beams. A calculation model for flexural bearing capacity was established, and finite element simulation and parametric analysis were carried out via ABAQUS. The test results show that steel bar corrosion reduces the yield and ultimate bearing capacity of RC beams by 8.54% to 12.12%, and decreases ductility by 5.9% to 49.1%, with corroded beams with low longitudinal reinforcement ratios being more prone to brittle failure. FRGC strengthening can significantly improve the performance of beams: after non-corroded beams are strengthened with single-layer FRGC, the yield load increases by 27.1% to 56%, the ultimate load increases by 34% to 42%, and the ductility coefficient increases by 34% to 42%, with the most prominent strengthening gain achieved in beams with low reinforcement ratios; after corroded beams are strengthened with FRGC, their flexural bearing capacity increases by 8.6% to 31.8%, and both the bearing capacity and ductility can be restored to the level of the original non-corroded beams; strengthening with double-layer fiber grid achieves higher bearing capacity but results in a certain decrease in ductility, while single-layer FRGC strengthening balances both performance and economic efficiency; an alkali content of 10 mol/L is confirmed as the optimal mix proportion for the FRGC matrix. The relative error between the calculated values obtained from the flexural bearing capacity calculation model established in this paper and the experimental values is less than 1.7%, and the finite element simulation results are in good agreement with the experimental results. The research findings can provide experimental basis and theoretical support for the FRGC strengthening design and engineering application of corroded RC beams in marine environments.