Plasmonic photocatalysts have opened up a new direction in utilization of visible light and promoting photocatalytic efficiency. An electrochemical deposition method is reported to synthesise metal@semiconductor (M@SC) core-shell nanocrystals. Due to the strong affinity of Au atoms to S2- and Se2- reduced at negative potential, CdS, CdSe and ZnS were selectively deposited on the surface of the Au core to form a uniform shell with a clear metal/semiconductor interface, which conquered the barrier caused by the large lattice mismatch between the two components. Plasmonic effects increased the photocatalytic performance, as well as provided a chance to in situ monitor the surface nucleation and growth. The structure formation process could be observed under dark-field microscopy (DFM) in real-time and precisely controlled via the scattering color, intensity and wavelength. The proof-of-concept strategy combines the electrochemical deposition and plasmonic imaging, which provides a universal approach in controllable synthesis of core-shell heterostructures, and leads to the improvement of plasmonic photocatalysts.