The article discusses the development of an innovative heterojunction system using non-stoichiometric Ni0·85Se co-catalysts anchored onto Mn0·4Cd0·6S solid solutions to enhance the efficiency of photocatalytic hydrogen evolution reaction (HER). The optimized Ni0·85Se/MCS hybrid achieved a remarkable HER rate of 42.5 mmol/g/h under visible light, showing a significant enhancement over pristine MCS. Through experimental characterization and Density Functional Theory (DFT) calculations, the study reveals that the work function difference between the materials facilitates directional electron transfer, while Ni sites on the catalyst surface optimize water adsorption and dissociation. The metastable Ni-Se configuration further regulates the energetics of key intermediates, enhancing charge separation efficiency and lowering the water dissociation energy barrier. The research aims to provide atomic-level insights into interfacial water activation mechanisms and establish guidelines for designing multifunctional co-catalysts in solar-driven hydrogen production systems. By constructing heterojunction structures like Ni0·85Se/MCS, the study demonstrates a promising approach to address challenges in photocatalytic hydrogen evolution and promote the application of sustainable hydrogen production technologies.