||We synthesized bispyrene molecule (Py-PEG) with excimer-monomer switching properties according to the viscosity of the medium. Because of the strong interaction between Pyrene, Py-PEG showed excimer fluorescence even in very dilute solutions. However, the formation of intramolecular dynamic excimers was restricted by the increase in the viscosity surrounding the molecules. At low viscosity, excited monomers could collide with other monomers in the molecule to form a dynamic excimer. As the viscosity gradually increased, the intramolecular conformational change in the medium was limited, making it difficult for the pyrene at both ends of the molecule to collide within the excitation lifetime. As a result, the excimer fluorescence decreased while the monomer fluorescence enhanced as the viscosity of the medium increased. Therefore, Py-PEG has potential to be used as an optical viscosity sensor, and the gelation process could be visually monitored through fluorescence change.