Abstract: The fluorescence decay dynamics of two kinds of 7-aminocoumarin derivatives molecules with intramolecular ratable groups in different mixed solvents was investigated by time-correlated single photon counting methods. The results show that the fluorescence lifetime does not change with viscosity in nonpolar mixed solvents, while it increases with the viscosity in polar mixed solvents. These viscosity effects proved that the excited states of coumarin molecules with unrigid structure can change into a twisted intramolecular charge transfer state, and there exists a potential barrier between the planar state and the twisted state