A simple and effective polymer fluorescent thermosensitive system was successfully developed based on the synergistic effect of excimer/monomer interconversion of pyrene derivatives and electrostatic interaction between polyelectrolyte and charged fluorophore. As for the system, the excimer-monomer conversion, thermosensitive behavior and thermo-responsive reversibility were investigated experimentally. Temperature variation and temperature-distribution induced fluorescence changes can be observed directly by naked eyes. Thus, this polymer system holds promise for serving as a fluorescent thermometer. A simple and effective polymer fluorescent thermosensitive system was successfully developed based on the synergistic effect of excimer / monomer interconversion of pyrene derivatives and electrostatic interaction between polyelectrolyte and charged fluorophore. As for the system, the excimer-monomer conversion, thermosensitive behavior and thermo-responsive reversibility were investigated experimentally. Temperature variation and temperature-distribution induced fluorescence changes can be observed directly by naked eyes. Thus, this polymer system holds promise for serving as a fluorescent thermometer.