A flexible fluorescent fiber temperature sensor doped with upconversion fluorescent nanoparticles based on fluorescence intensity ratio (FIR) technology is proposed to improve the stability and anti-interference of contact flexible fiber temperature sensor. The composite flexible fiber doped with rare earth ions emits stable fluorescence when excited, and the intensity of the central fluorescence peak corresponding to the thermal coupling energy level of upconversion nanoparticles follows the changes of temperature. The proposed flexible fiber temperature sensor mainly uses the ratio of the center fluorescence peak intensity corresponding to the thermal coupling energy level of the doped Er³⁺ after FIR treatment as the characterization value. Its temperature response is thermally enhanced which means the fluorescence intensity is enhanced with the increase of temperature. The experimental results show that the proposed sensor exhibits high stability and strong anti-interference, as well as good flexibility and deformation capability, high sensitivity and repeatability. The maximum absolute sensitivity and the maximum relative sensitivity are 0.0038 ℃^-1 and 1.29 %/℃, respectively.