Abstract: Fluorescence Correlation Spectroscopy (FCS), a kind of single molecule technique, performs temporally correlative analysis of spontaneous intensity fluctuation of fluorescence from thermal equilibrium with direct measurements of the diffusion coefficient and the concentration of fluorescence molecules in aqueous solution. Some factors, such as the quantum coefficient, the measurement time, the refractive index of the sample and the temperature deviation , make effects on the measurement accuracy. In the thesis, FCS is used to measure the viscosity of the glycerin-water solution with Rhodamine 6G and Cy5, respectively and the experiment results show that the quantum coefficient plays an important role in the measurement accuracy with 1000 photons/sec as the minimal level.