Metal/semiconductor composites are promising catalysts with superior catalytic activity. In this work, a Cu/CuO-Ag composite with structure that consisted of Ag and CuO nanoparticles (NPs) decorated on the surface of Cu were fabricated via a facile in situ method. With characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and inductively coupled plasma atomic emission spectrometry (ICP-AES), the structure and components of the Cu/CuO-Ag composite were well-defined. The Cu/CuO-Ag composite exhibited superior catalytic activities for the reduction of 4-nitrophenol (4-NP) in the presence of NaBH4 with just a trace amount of Ag NPs (1.28 wt %). The reduction reaction is completed in 75 s with an apparent rate constant kapp of 4.60 × 10-2 s-1. The Cu/CuO-Ag composite also showed excellent durable catalytic stability, as no significant activity loss was detected in the consecutive five reaction runs. With the aid of the Sabatier principle and volcano plot, the opportune chemical adsorption energy of the reagent 4-NP on the Cu/CuO-Ag composite was inferred to be the key to its high reaction rate. The CuO NPs as a semiconductor with narrow band gap also could help the Cu/CuO-Ag composite to capture the electrons/hydride ions and increase opportunities for 4-NP to be reduced. Furthermore, the Cu/CuO-Ag composite exhibited outstanding activity on the oxidative degradation of methylene blue (MB). This work enriched the bimetal/semiconductor catalyst system and supplied new insight into the catalysis mechanism.