High-strength aluminum alloys, such as Al-Cu-Li and Al-Zn-Mg, are essential structural materials in aerospace, manufacturing, transportation, and mobile communication owing to their excellent strength-to-weight ratio. However, their use in critical applications is significantly limited by hydrogen embrittlement (HE), a phenomenon in which H atoms interact with microstructural features such as grain boundaries (GBs), leading to irreversible degradation of mechanical properties and potentially catastrophic failures. Despite extensive research, the atomic-scale mechanisms of H trapping at GBs and their detrimental effects on GB cohesion remain unclear, impeding the development of effective anti-embrittlement strategies. This study utilizes first-principles calculations to investigate these issues, aiming to provide a theoretical foundation for anti-embrittlement engineering. The results indicate that H atoms are most stably adsorbed at the short-bridge site on the Al (001) surface, with an adsorption energy of -3.051 eV, and tend to occupy tetrahedral interstitial sites (TIS) in the matrix. The diffusion path of H atoms into the matrix follows the TIS-OIS-TIS mechanism (where OIS denotes the octahedral interstitial site), with significant migration barriers of 0.32-0.56 eV, suggesting a challenge for H penetration into the matrix. Notably, Mg and Zr atoms spontaneously segregate to Site 1 of the Al Σ3(111)[110] GB; however, their effects are different: Mg weakens GB cohesion through charge depletion, whereas Zr significantly strengthens GBs by inducing high charge density, strong electronic localization, and d-p orbital hybridization. In addition, Zr segregation not only traps H atoms with a minimum trapping energy of -0.459 eV but also effectively suppresses H-induced damage to adjacent Al-Al metallic bonds, preserving GB strength. By contrast, Zn segregation has limited strengthening effects on GBs and may even facilitate H trapping. This study clarifies, at the atomic scale, that Zr enhances HE resistance through dual mechanisms: reinforcing GB cohesion and inhibiting H-induced degradation.