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Microwave hyperthermia cure is an effective method in malignant knub cure and the key of this technique are heating and measuring temperature accurately. For the needs of heating and measuring temperature accurately, a kind of fluorescent lifetime optical fiber temperature sensor was introduced in this paper. Based on the fluorescent lifetime mechanism which means temperature measured is depended on fluorescent lifetime only, the sensor is to measure temperature by measuring the fluorescent lifetime accurately. Phase-locked technique is used to measure the fluorescent lifetime. The phase-locked technique based on double reference pulse signals modulates the frequency of incentive-light and measure lag angle between the fluorescence and incentive-light. Frequency of Voltage Controller Oscillator (VCO) has something to do with the lag angle, as long as the lag angle exists, there is voltage importing VCO and exporting frequency, and then this frequency go to modulate the frequency of incentive-light till the lag angle is zero. Finally, the fluorescent lifetime is gained by measuring the frequency of VCO to get the temperature measured when the lag angle is zero. A kind of probe used in the sensor was designed too. The probe is small in size, safe and convenient to insert body, at the same time can avoid interference of electromagnetic radicalization. As a result, it is suitable for medical cure field. A lot of experiments have been carried out and the results show that the sensor's measurement precision is Â±0.05Â°C, temperature resolving power is 0.01Â°C and response time is within 0.1 second between 20Â°C and 50Â°C. It will play an important role in medical temperature measurement and have a wide application in the future.