In the realm of photoenergy conversion, the scarcity of efficient light-driven semiconductors poses a significant obstacle to the advancement of photocatalysis, highlighting the critical need for researchers to explore novel semiconductor materials. Herein, we present the inaugural synthesis of a novel semiconductor, CdNCN, under mild conditions, while shedding light on its formation mechanism. By effectively harnessing the [NCN]2⁻ moiety in the thiourea process, we successfully achieve the one-pot synthesis of CdNCN-CdS heterostructure photocatalysts. Notably, the optimal CdNCN-CdS sample demonstrates a hydrogen evolution rate of 14.7 ​mmol ​g−1 ​h−1 under visible light irradiation, establishing itself as the most efficient catalyst among all reported CdS-based composites without any cocatalysts. This outstanding hydrogen evolution performance of CdNCN-CdS primarily arises from two key factors: i) the establishment of an atomic-level N-Cd-S heterostructure at the interface between CdNCN and CdS, which facilitating highly efficient electron transfer; ii) the directed transfer of electrons to the (110) crystal plane of CdNCN, promoting optimal hydrogen adsorption and active participation in the hydrogen evolution reaction. This study provides a new method for synthesizing CdNCN materials and offers insights into the design and preparation of innovative atomic-level composite semiconductor photocatalysts.