Abstract: To search after for the dependence of the emission fluorescence on the excitation rate and the spontaneous-radiation rate, the rate equation of the upper level population for two-level system was derived and it was solved on certain. conditions. The variation regularities of the aforementioned population and the photon-number of the emission fluorescence were presented. The fluorescence photon-number is proportional to the excitation rate while the spontaneous-radiation rate has an optimum decided by a trigonometric equation.