Abstract: NiCoP/NiOOH heterojunctions serve as non-precious metal single-atom catalytic materials with potential applications in alkaline hydrogen evolution reaction (HER), but the regulatory mechanism of NiCoP component layer thickness on the catalytic performance of alkaline HER lacks theoretical study. Based on density functional theory (DFT), this work systematically investigates the structural stability and HER catalytic performance of NiCoP(100)/NiOOH(001) heterojunctions with different layer thicknesses. The results show that with increasing NiCoP(100) layers, the stability and catalytic activity of the heterojunction gradually decrease. Among them, the heterostructure with two NiCoP(100) layers achieves the optimal balance of interfacial electronic effects and surface reconstruction effects. This study reveals the thickness-dependent structure-activity relationship of the heterojunction, providing important theoretical guidance for the rational design of efficient alkaline HER electrocatalysts.