A new type of fluorescence fiber optic temperature sensor based on the detection of fluorescence intensity ratio (FIR) is proposed. Benefited from the temperature-dependent characteristic of upconversion luminescence (UCL), it can be applied in fiber optic temperature sensors. Rare earth doped upconversion nanoparticles (UCNPs) NaYF4:Er3+,Yb3+ are embedded in a multi-mode quartz fiber through the technology of fiber fusion as the sensing unit of temperature sensors. A 980 nm laser is used to stimulate UCL in a temperature range from 40 °C to 100 °C. Experimental validation and spectral analysis are carried out to confirm the rationality of sensors' design. Results show that FIR changes with the temperature in Boltzmann distribution law. The sensitivity of the temperature sensors can reach the value between 0.0087 and 0.0144 K-1.