Aimed to enhance the light-to-heat energy transfer of nanofluid-based volumetric solar thermal collectors, it is necessary to obtain wide spectrum absorption materials. In this paper, fusiform copper oxide (CuO) nanoparticles are prepared and silver (Ag) nanoparticles with localized surface plasmon resonance (LSPR) effect are interspersed on the surface of CuO nanoparticles just like core/shell nanostructure. Nanofluids based on CuO and CuO/Ag composite nanoparticles are compared under different concentrations. Due to LSPR effect excited on the Ag surface, fusiform CuO/Ag plasmonic nanoparticles have remarkable broad-band absorption with three absorption peaks in the visible and near-infrared region compared to CuO nanoparticles. The photo-thermal conversion efficiency of CuO/Ag plasmonic nanofluid is higher than that of CuO nanofluid under the same conditions, and is proportional to nanoparticles concentration, among which CuO/Ag nanofluid with the volume fraction of 0.025% has the maximum instantaneous photo-thermal conversion efficiency, especially 96.11% at 35 degrees C and 39.58% at 60 degrees C.