Abstract: In this study, the pathogen was obtained from the tea leaves suffering from tea anthracnose by tissue isolation method and identified. Moreover, nano-Cu(OH)₂ was prepared by the precipitation method and characterized and the inhibitory activity and inhibition mechanism of nano-Cu(OH)₂ on the tea anthracnose pathogen were investigated. The results show that the pathogen isolated from tea leaves clustered with Colletotrichum siamense on the same branch with a self-support rate as high as 91%, confirming the isolated pathogen was C. siamense. The morphology of synthesized nano-Cu(OH)₂ were petal-like, bunchy, and clustered nanoparticles, accompanied by an aggregation phenomenon. The highest inhibition rate of 99.16% to C. siamense was achieved at 2 days of treatment 300 mg/L nano-Cu(OH)₂ treatment. The Zeta potential of C. siamense was (−8.91 ± 1.80) mV and that of nano-Cu(OH)₂ was (12.55 ± 1.33) mV. The strong antimicrobial effect of nano-Cu(OH)₂ might be due to electrostatic interactions. After treatment with nano-Cu(OH)₂, the fungal hyphae showed obvious protrusions and distortions, severe damage, and ruptured fungal cell membranes. Concurrently, a significant amount of reactive oxygen species was generated, resulting in the death of most cells. DNA damage measurements showed that nano-Cu(OH)₂ treatment caused the tea anthracnose pathogen DNA bands narrower and darker, and as the concentration of nano-Cu(OH)₂ increased, the DNA concentration decreased. The results of antioxidant enzyme activity and malondialdehyde (MDA) content measurements showed a decrease in superoxide dismutase (SOD) activity and an increase in MDA content in the fungi. In conclusion, these results suggest that nano-Cu(OH)₂ can induce pathogenic cell membrane damage, DNA damage, inhibition of SOD activity, and promotion of MDA production, ultimately leading to cell death. These findings would provide a new approach for the effective control of tea anthracnose using nano-pesticides.