A highly efficient photocatalyst, designated mesoporous crystalline carbon nitride (MCCN-AT), was synthesized via molten-salt-assisted thermal polymerization of 3-amino-1,2,4-triazole using a silica-templating strategy. Structural analyses by XRD, FT-IR, and NEXAFS confirmed the formation of a crystalline carbon nitride framework composed of poly(heptazine imide) units. The resulting material exhibited a high surface area, well-developed mesoporosity, and an optimized optical bandgap, enabling strong light absorption and charge transport. Under simulated solar irradiation, MCCN-AT demonstrated markedly enhanced hydrogen evolution activity compared to conventional bulk graphitic carbon nitride. This superior performance is attributed to efficient charge separation and the abundance of accessible active sites within the mesoporous channels. Time-dependent density functional theory (TD-DFT) calculations further revealed favorable excited-state properties, including efficient H⁺ adsorption and H₂O dissociation pathways, consistent with the experimental results. Overall, this work presents a rational strategy for designing crystalline carbon nitride photocatalysts with optimized structural and electronic characteristics for sustainable solar-driven hydrogen production.