Glypican-3 (GPC3) might be used as new biomarker of liver cancer for the development of new diagnostic methods. The most commonly used methods for protein detection are based on natural enzymes, which are easily affected by environmental conditions and suffer from the rigorous preparation conditions. Thus, the development of new enzyme mimetics with high and stable catalytic activity is of great significance in diagnostic applications. In this paper, copper ions (Cu²⁺) was found to possess the peroxidase-like catalytic activity, which can catalyze H₂O₂-mediated oxidation of peroxidase substrate and obtain the oxidation product with color change. This catalytic activity is much more stable than other nanomaterials based peroxidase mimetics, and can significantly increase by increasing the concentration of H₂O₂. It is worth mentioning that the absorbance signal induced by 5 nM Cu²⁺ can be easily detected. This Cu²⁺-catalyzed reaction can be also applied in the detection of GPC3 by using the anti-GPC3 antibody functionalized CuO NPs, which can release the Cu²⁺ by dissolved in HCl solution. This method permits detection of as low as 0.26 pg mL⁻¹ GPC3. This sensitivity is about one or several magnitudes higher than that of ELISA or other peroxidase mimetics based methods. The high catalytic activity of Cu²⁺ and the signal amplification process of CuO NPs into high amount of Cu²⁺ also make this method more simple and effective.