Carbon quantum dots have attracted much attention in the fields of bioimaging, biolabeling and drug delivery. Theoretical and experimental studies have shown that carbon quantum dots are expected to show unique optical properties due to their quantum confinement and edge effect. In this report, water-soluble and color-tunable fluorescent carbon dots were prepared by cyclic voltammetry (CV) in alkaline condition. The structure of the carbon dots was confirmed by means of transmission electron microscope (TEM), Raman spectrum and atomic force microscope (AFM). The finding shows that the carbon quantum dots have a uniform diameter around 19 nm, and are mainly consist of 1 to 4 layers of graphene with a mass of oxygen-containing functional groups. Their features and properties were characterized by photoluminescence spectra (PL), UV-visible spectroscopy (UV-vis), fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results indicated that the carbon quantum dots have two fluorescent emission peaks at 400 nm and 525 nm. The former peak was associated with the π-π conjugated system of carbon dots, which showed an excitation-wavelength dependent feature, while the latter peak was associated with n-π conjugated system of oxygen-containing functional groups, which remained unshifted when excited by different excitation wavelengths, suggesting a novel kind of fluorescent feature and mechanism different from those of previously reported carbon quantum dots depending on excitation wavelengths. The fluorescent color of carbon quantum dots could be controlled by the cycles of CV. As the increase of CV cycles, the relative content of oxygen-containing functional groups increased, leading to the decrease of the relative intensity of fluorescent peaks at 400 nm and the increase of the relative intensity of fluorescent peak at 525 nm; and the color of the fluorescent changes from blue to yellow. The color-tunable fluorescent carbon quantum dots showed a high water solubility and good photostability, which indicates that the carbon quantum dots might be used as a bioimaging marker in cell imaging.