N and S co-doped carbon dots with bright blue emission have been synthesized by a facile one-pot hydrothermal treatment. A specific compound, guanidine thiocyanate via a hydrogen bond to form the molecule unit, was chosen as the dopant. In the structure characterization, N and S elements could be sufficiently doped by means of the heteroatom or the functional groups bonded on the surface of carbon dots. The results of the photoluminescence spectra indicated a change of emission process from excitation-independent to excitation-dependent with the optimal excitation wavelength at 395 nm, suggesting that some small fluorophore molecules exist in citric acid-derived carbon dots solution and bond on the surface of the carbon dots after carbonization in view of the triexponential feature of fluorescence lifetimes of carbon dots. Now that so many fluorophores attach on the carbon dots surface, the as-prepared carbon dots were used as a sensing probe for the detection of silver ion. As expected, the changes of fluorescence intensities were linearly proportional to the different concentration ranges of silver ion. The －S－C≡N group could accelerate the quenching of carbon dots towards silver ion. The facile preparation method and high performace of as-prepared carbon dots provide a possible approach for efficient detection of silver ion for the application in industrial pollutants.