In this paper, a refractive index insensitive PbS fiber temperature sensor based on Sagnac interferometer is proposed. Firstly, the sensing mechanism of refractive index and temperature is analyzed. Then a large amount of sensors with different length are fabricated. Comparing the transmission spectrum of these sensors, the PbS fiber with the length of 4mm and 10mm are chosen for sensing experiments. Finally, the experiment platform is established to measure temperature and refractive index. Under the condition of different temperature and refractive index, the relationship between the interference spectrum, temperature and refractive index is analyzed, respectively. In temperature experiment, the temperature range is 30℃-65℃. The experimental results show that the interference spectrum is red shifted. When the PbS fiber length is 4mm, the temperature sensitivity can be up to 78.23pm/℃. When the PbS fiber length is 10mm, the temperature sensitivity is 59.16pm/℃. The refractive index range is 1.3525-1.4505 corresponding to the glycerite solution concentration of 10%-80%. The experimental results show that there is no wavelength shift in the interference spectrum. So this PbS fiber sensor is almost insensitive to refractive index.