This paper reports a method of fabricating the Cu₂O-Ag SERS substrate by in-situ growth on a copper sheet. The as-fabricated substrate has considerable Raman enhancement after we optimized the annealing temperature and time for Cu₂O fabrication and the AgNO₃ concentration and reaction time for Cu₂O-Ag fabrication. In addition, the concave spaces on the substrate surface and the uniformly distributed Ag NPs provided rich SERS “hot spots”. The substrate also has good hydrophobicity, homogeneity, stability, and sensitivity, and it can detect R6G with the limit of detection (LOD) of 0.78 nM. Furthermore, the substrate has great sensitivity towards several prohibited drugs, and the Raman intensity has an excellent quantitative relationship with drug concentration. To be specific, the LODs for malachite green, enrofloxacin, and nitrofurazone are 4.9 nM, 0.72 μM, and 0.12 μM, respectively. In conclusion, the proposed method has a simple process, low costs, and high SERS activity, indicating bright application prospects for environmental monitoring.