Preparation of alkali metal doped g-C₃N₄ materials is an important branch in the research of g-C₃N₄ semiconductor photocatalytic materials. However, there is still lack of study on g-C₃N₄ materials revealing mechanisms in photosensitizer-assisted photocatalytic degradation. In this study, Na⁺ doped g-C₃N₄ photocatalysts (Na⁺/g-C₃N₄) were prepared using solution synthesis, calcination, and solvothermal reaction methods.The doped position of Na⁺ in g-C₃N₄ and photoelectric performance were determined. The changes of morphological, specific surface area, and pore size of Na⁺/g-C₃N₄ materials were analyzed by scanning electron microscopy, N₂ adsorption and desorption experiments. In Na⁺/g-C₃N₄ materials, the Na⁺ loaded in a cyclic structure composed of three heptazine structural units, coordinating with N atoms. Na⁺/g-C₃N₄ changed the adsorption performance of g-C₃N₄, altered its bandgap width and position of conduction (valence) band, and increased its separation rate of photogenerated electrons and holes and charge transport rate of the material by affecting the π-conjugated system of g-C₃N₄. During the solvothermal reaction process for synthesis of Na⁺/g-C₃N₄, strong hydrolysis caused decomposition of unstable structures of g-C₃N₄ while the C-O^- bonds were formed at the edge of g-C₃N₄. The physical and chemical adsorption sites for methylene blue (MB) of Na⁺/g-C₃N₄ materials are confirmed by π-conjugated system and C-O^- bonds of Na⁺/g-C₃N₄, by which Na⁺/g-C₃N₄ materials can adsorb MB up to 93.25%, in contrast to the g-C₃N₄ materials’ adsorbtion only up to 24.50%. Under visible light irradiation, due to their strong adsorption capacity and photosensitivity to MB, Na⁺/g-C₃N₄ materials have constructed a unique photosensitive- photocatalytic degradation system with MB. MB not only acts as the photosensitizer for self degradation but also collaborates with Na⁺/g-C₃N₄ materials for photocatalytic degradation. At pH 6.0, the maximum degradation rate of MB is up to 96.40% in the photosensitive-photocatalytic system constructed with MB and Na⁺/g-C₃N₄ samples.