This paper presents and demonstrates a novel in-fiber integrated high sensitivity temperature sensor based on long Fabry-Perot (FP) resonator. In a quartz capillary, the FP resonator was composed of two single mode fibers (SMFs) whose end faces were coated by gold films. The temperature can be obtained by measuring the length variation of the FP cavity. A white light interference demodulation system was used to measure the length variation of the FP cavity. By the multiple reflections of light in the FP cavity, we achieved the sensitivity multiplication of the sensor. The proposed sensor measured the temperature up to 350°C for 2 hours, and the sensitivity of the sensor is six times that of the traditional interference temperature sensor. Due to the advantages of low cost, high sensitivity and simple fabrication, this temperature sensor can be widely used in high temperature applications.