The fabrication of a multifunctional sensor together with a widening temperature-sensing range is an essential challenge in optical thermometers especially for trivalent lanthanide-doped materials. Herein, we design a wide range, highly sensitive, and multifunctional thermometer by exploiting the emission spectrum of Eu3+ ions, and further detailed discussion has been made on the new temperature-sensing mechanism. The sensor can be operated between 358 and 548 K with a maximum relative sensitivity (S-r) of 0.93% at 358 K, which is higher than that of most temperature-sensing materials. A paramount superiority is that the calibration parameter can be directly calculated from the single Eu3+ emission spectrum, avoiding the demand of other calibrations, which realizes the coexistence of a simple structure and high precision. Furthermore, other up-conversion thermometers based on Er3+/Ho3+/Yb3+ co-doped Ba2TiGe2O8 (BTG) phosphors as well as the down-conversion thermometer based on Eu3+-doped Ba2TiGe2O8 (BTG:Eu3+) phosphor have been synthesized for comparison, and the results show that the novel thermometer (BTG:Eu3+) has a much higher sensitivity than that of the traditional thermometers (BTG:Er3+/Ho3+/Yb3+). In addition, the versatility of the phosphor (BTG:Eu-3(+)) is simultaneously reflected in its applications to red phosphor for white-light emitting diodes (W-LEDs) and plant growth lamps. All of the results suggest that BTG:Eu3+ could be a good candidate with its highly sensitive S-r value for optical thermometry and as a safety sign in high-temperature environments.