Poly(p-phenylene benzobisoxazole) (PBO) fibers have garnered significant attention due to their exceptional mechanical properties, high thermal stability, and flame resistance, demonstrating broad application prospects in aerospace, transportation, and new energy sectors. However, the aging behavior of PBO fibers in humid environments severely compromises their long-term service performance, limiting their development and application in high-tech fields. Therefore, enhancing the anti-aging properties of PBO fibers under humid conditions remains a critical challenge. In this study, the protective effect of graphene coatings on PBO fibers in humid environments was investigated using single-fiber micro-tensile testing. The results indicate that PBO fibers undergo hydrolysis in humid environments, leading to a reduction in Young’s modulus and tensile strength. While coating PBO fibers with covalently cross-linked graphene oxide significantly enhances their mechanical properties, it fails to improve their moisture resistance. Further, the reduction of covalently cross-linked graphene oxide through high-temperature pyrolysis yields reduced graphene oxide with a dense layered stacking structure and excellent hydrophobicity, effectively improving the anti-aging performance of PBO fibers. The reduced graphene oxide-coated PBO fibers exhibit a strength retention rate of nearly 98.2% at 50% relative humidity and up to 96.4% at 80% relative humidity.