Regenerated fiber Bragg grating (RFBG) cannot be directly used in practical engineering for temperature measurement due to its low reflectivity, so a high-temperature fiber laser sensing method based on low-reflectivity RFBG is proposed. RFBG is used as one mirror of the resonant cavity, and a length of unpumped Er-doped fiber(EDF) as saturated absorber is adopted to compress line width and suppress multi-longitudinal mode of laser. The current threshold of fiber laser is 68.9 mA. At 150 mA, the output laser is stable and has a good linear relationship with temperature when temperature varies in the range of 300 to 800 ℃. In the temperature rising and falling test, the correlation coefficient is up to 0.99974, and the average temperature sensitivity is 15.41 pm/℃. During 3 h of laser stability test at 700 ℃, the maximum variation of the laser wavelength and intensity is 0.032 nm and 0.409 dB, respectively. The results show that the signal-to-noise ratio is higher than 50 dB, the output laser is stable, and there is no mode hopping phenomenon in the temperature-rising and temperature-falling process.