Abstract: The rapid development of industry has led to substantial discharge of organic dyes into aquatic systems, posing severe threats to ecological environments and human health. To address persistent water pollutants, developing photocatalysts with low energy consumption, high efficiency, and long-term stability has become a critical challenge in degradation technology applications. In this study, a calcination-hydrothermal synergistic approach was employed to in situ grow Mn-doped Sn₃O₄ nanostructures on g-C₃N₄ nanosheets, successfully constructing a II-Type Mn-Sn₃O₄/g-C₃N₄ heterojunction photocatalyst for Rhodamine B (RhB) photodegradation. By varying the Mn-Sn₃O₄ loading content, a series of Mn-Sn₃O₄-x/CN composites with different compositions were obtained. Comprehensive characterization confirmed the successful fabrication of the catalysts and bandgap optimization. Photocatalytic evaluation through RhB degradation demonstrated that the Mn-Sn₃O₄-10/CN composite achieved 82.5% degradation efficiency within 40 minutes under illumination, significantly outperforming pristine g-C₃N₄. This enhancement verifies the synergistic effect between II-Type heterojunction formation and Mn doping. The rationally designed Mn-Sn₃O₄-x/CN composite exhibits outstanding stability and degradation performance, providing valuable guidance for developing advanced g-C₃N₄-based photocatalysts.