High-quality Au@CdS core-shell nanoparticles (CSNs) have been synthesized to improve photo-conversion efficiency in photocatalysis. They demonstrate higher photocatalytic activity in the experiment of photocatalytic degradation of rhodamine 6G (R6G) solution than that of CdS counterparts. Au@CdS CSNs can broaden the absorption range in visible region compared to CdS counterparts. The heterojunction interface between Au and CdS facilitates the separation of photo-generated electron-hole pairs, and transfers electrons from CdS region to Au core. The two advantages are crucial to improve the photocatalytic activity of Au@CdS CSNs. Charge transfer mechanism between metal and semiconductor is efficient that can be used to guide the design of photocatalysts, photovoltaics, and other optoelectronic devices to effectively utilize the solar power. In this paper, we research the photocatalytic process by surface-enhanced Raman scattering (SERS). The combination of photocatalysis and SERS not only can show the change in concentration of R6G solution, but also can provide the information of the change of R6G molecular structure in photocatalytic process.