Plasmonic imaging under a dark-field microscope (DFM) holds great promise for single-particle analysis in bioimaging, nanophotonics, and nanocatalysis. Here, we designed a DNA-directed programmable assembly strategy to fabricate a halo-like Au nanostructure (nanohalo) that couples plasmonic large gold nanoparticles (L-AuNPs) with catalytically active small AuNPs (S-AuNPs) in a single nanoarchitecture. Catalytic reaction occurring on S-AuNPs changes its permittivity, which results in a significant variation of the plasmonic resonance of the nanohalo. Hence, we can indirectly monitor catalytic reactions on a single nanohalo under DFM, on the basis of which we have obtained quantitative information on both nanocatalysis and catalyst poisoning. Our study thus provides a cost-effective means to quantitatively study metal NP-based catalysis at single-particle level.