<p>The band structure of the photocatalysts is recognized as a critical factor in photocatalytic nitrogen fixation. In this study, we developed a simple strategy to load Ru nanoparticles on Mo-MOF-Me with a [Mo₈O₂₆(1-Meim)₂]⁴⁻ structure (1-Meim=1-methylimidazole). Both Ru⁰ doping level and Mo⁵⁺ defect level were introduced into the band gap of Mo-MOF-Me, extending visible-light absorption to 700 nm, attributed to the electron transition from Mo⁵⁺ defect level to Ru⁰ doping level. Ru@Mo-MOF-Me exhibits significant enhancement in photocatalytic nitrogen fixation performance compared to Ru@Mo-MOF, owing to the strong electron-donating ability of the methyl group in 1-methylimidazole ligand, resulting in a higher amount of Mo⁵⁺ and a higher valence band, which generates a higher Ru⁰ energy level with energetic electrons as the active centers. Moreover, the Ru⁰ energy levels are lower than the conduction band (CB) of Mo-MOF-Me, accelerating photogenerated electron transfer from the CB of Mo-MOF-Me to Ru⁰ to improve nitrogen reduction activity. This work establishes a clear relationship between the photocatalytic activity and the band structure of Ru@Mo-MOF-Me. These findings provide critical insights into the band structure engineering and underscore the importance of constructing metal-semiconductor heterojunctions for efficient photocatalytic nitrogen fixation.</p>