Fluorescence optic-fiber temperature sensor based on Planck law is studied by using sapphire fiber as thermal probe. With a high melting point,over 2000 ℃,high transmissions in the wave band of 0.3-0.4 μm and favorable mechanical and chemical inert,sapphire fiber is very attractive for high temperature optical fiber sensor and near infrared energy delivery. The measured fluorescence life time can be obtained by calculating tangent value of first non-zero phase angle and using fast Fourier transform to measure the life time of fluorescence. This method has virtues such as fast speed,small error and no background interference. In the detection range from 0 to 450 ℃,the thermometer has an average temperature resolution of 0.4 ℃. The experimental results show that this system can effectively decrease the influence of instability of light source and intensity variation in measuring channel on results. The system has advantages of high accuracy,very high temperature resolution,good dynamic response and immunity to the electro-magnetic interference.