Surface vacancies, serving as the activation centers for surface-adsorbed species, have been widely applied in catalysts to improve their activity and selectivity. In the case of ternary compound semiconductors, there is some controversy about exposed atoms and surface defects. Two-dimensional layered BiOCl is an important photocatalyst, which has had numerous studies focused on its oxygen vacancy (OV) and bismuth vacancy (BiV). It has been realized that its (001) surface can consist of exposed halogen atoms rather than oxygen atoms, which thus needs a new explanation for its surface defect engineering mechanism. Using first-principles calculations, the activation behavior of NOX (NO2, NO, N2O) at a chlorine vacancy (ClV) on the BiOCl (001) surface is systematically studied. It is found that after introducing ClV on BiOCl (001) surfaces, NOX molecules all show excellent activities with longer chemical bonds by capturing electrons from the catalyst. Our work furnishes fundamental insight into the activation of small molecules on defect-rich surfaces of ternary compound catalysts.