To overcome the limitations of traditional photocatalysts, such as inefficient separation of charge carriers and poor visible-light absorption, S-scheme g-C₃N₄/TiO₂ heterojunction photocatalysts were synthesized via a combined method of thermal polymerization, hydrothermal synthesis, and calcination. The crystal structures, morphological features, and optical properties of the composites were systematically characterized, and their photocatalytic performance was evaluated through tetracycline (TC) degradation and hydrogen evolution experiments. Trapping experiments and electron paramagnetic resonance (EPR) measurements were conducted to elucidate the reaction mechanisms. The results demonstrate that the S-scheme heterojunction effectively extends the visible-light absorption range and facilitates the efficient separation of photogenerated electron-hole pairs. Under optimal conditions, the composite achieved a TC degradation rate of 94.5% and a hydrogen evolution rate of 329.1 μmol·h^-1·g^-1 after 8 h of irradiation, both values being significantly higher than those of pristine g-C₃N₄ or TiO₂. Moreover, the S-scheme g-C₃N₄/TiO₂ heterojunction retained high photocatalytic activity over five consecutive cycles, confirming its excellent stability. Mechanistic investigations revealed that the S-scheme heterojunction maintained strong redox capacities, with superoxide radicals (·O₂^-), hydroxyl radicals (·OH), electrons (e^-), and holes (h⁺) serving as the primary active species responsible for TC degradation and H₂ production.