This study presents a novel approach utilizing the concentration gradient of thiourea dissociation products to synthesize a CdNCN-CdS composite photocatalyst with an atomic-level heterostructure (NCN-Cd-S). The strong electron affinity of CdNCN and efficient electron transfer at the interface enhance photocatalytic hydrogen evolution, achieving a record-high rate of 14.7 mmol·g⁻¹·h⁻¹ under visible light, surpassing other CdS-based composites.