The invention discloses an application of carbon nanodots as a water-soluble ratiometric fluorescent probe and solves the problems that the existing ratiometric fluorescent probes are high in preparation cost, poor in luminescent stability and provided with potential toxicity. According to the application, the carbon nanodots are dissolved in a solution to be measured, an excitation wavelength is fixed at 360-400nm,the solution to be measured dissolved with the carbon nanodots is subjected to fluorescence spectrum detection at a fixed temperature, intensities of a blue maximum fluorescence-emission peak and a green maximum fluorescence-emission peak are measured, the ratios of the blue maximum fluorescence-emission peak intensity and the green maximum fluorescence-emission peak intensity are calculated, so that the detection of the objective to be measured is achieved. The application has the advantages of being low in cost, good in luminescent stability, non-toxic, high in sensitivity and capable of achieving detection for temperature, pH of water solution, ethanol concentration in ethanol water and Fe3+ concentration in Fe3+-containing neutral water solution simply, accurately and quickly.