A kind of fluorescence optic-fiber temperature sensor is studied. It used sapphire fiber thermal probe, making use of the method that FFT transformation to carry on a processing to the fluorescence signal. Among the detection range from 0 to 450°C, the thermometer has an average temperature resolution of 0.4°C. The experimental results show that this temperature measuring system can effectively decrease the influence of instability of light source and intensity variation in measuring channel on measurement results. The system has the advantage of high accuracy, very high temperature resolution, good dynamic response and immunity to the electro-magnetic interference. So it may be used in microwave treatment and thermal monitoring of Medium Voltage Substations.