In this paper, bright and high-quality carbon dots (CDs) were prepared by refluxing a membranous carbonized β-cyclodextrin in nitric acid. The structure and optical properties of the CDs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), fluorescence spectrophotometry (FL), X-ray diffraction (XRD) and Raman spectroscopy. The mechanism for the formation of the CDs is also discussed. As-prepared CDs could be used as peroxidase mimics for the detection of H₂O₂. The results indicate that the CDs are capable of rapid and sensitive detection of H₂O₂ in the linear range from 2 × 10⁻⁶ to 5 × 10⁻⁴ mol L⁻¹ and the limit of detection of H₂O₂ is as low as 1 × 10⁻⁶ mol L⁻¹. More importantly, the peroxidase-like activity of the CDs is expanded to the detection of some oxidative ions (such as Fe³⁺ and Ag⁺). The linear ranges for Fe³⁺ and Ag⁺ were 8 × 10⁻⁶ to 1 × 10⁻⁴ mol L⁻¹ and 5 × 10⁻⁶ to 1 × 10⁻⁴ mol L⁻¹ respectively, with corresponding detection limits of 8 × 10⁻⁷ mol L⁻¹ and 5 × 10⁻⁷ mol L⁻¹ respectively.