Fluorescence optical-fiber temperature sensor based on Planck’s law is studied in this paper. Sapphire fiber is adopted as thermal probe. With steady physical chemistry performance, higher melting point, low loss in the waveband of 0.3~0.4 μm , sapphire fiber is very attractive for temperature optical fiber sensing and near infrared energy delivery. Making use of the method of FFT transformation to carry on measurement of fluorescence life time, the measured fluorescence life time can be obtained by tangent value of first non-zero phase angle. This method has virtues such as fast speed, small error and no background interference. Among the detection range from 0 to 450 , the thermometer has an average temperature resolution of 0.4°C .The experimental results show that the temperature measuring system can effectively decrease the influence of instability of light source and intensity variation in measuring channel on measurement results. This system also has advantages of high accuracy, very high temperature resolution, good dynamic response and immunity to the electro-magnetic interference.