In terms of temperature sensing, rare earth ion-doped upconversion luminescent materials have many outstanding advantages, such as anti-interference, fast response, and high sensitivity. In this work, Er³⁺/Yb³⁺-doped LaGaO₃ microparticles have been prepared through a high-temperature solid-phase method. The microcrystals show excellent upconversion luminescence and temperature sensing under 980 nm excitation. The maximum relative sensitivity is 1.1670 × 10⁻² K⁻¹, and the temperature deviation does not exceed 1.2 K. Moreover, an optical fiber temperature sensor has been developed based on rare earth ion-doped microcrystals. The fabricated sensing device could be used in monitoring human body temperature. Our experimental results suggest that the optical fiber temperature sensor constructed with Er³⁺/Yb³⁺ codoped LaGaO₃ microcrystals has high precision and stability. The proposed sensing device has a wide range of application prospects in temperature detection.