In this study, the temperature sensing system performances of a fiber loop ring-down cavity with a fiber Bragg grating (FBG) are investigated by analyzing the spectral response of an FBG and the dynamic variation of an exponential decay. A theoretical FBG loop ring-down cavity model is established to investigate the relationships of the central wavelength of the FBG transmission spectrum and the exponential decay time with temperature change as well as their dynamic evolution procedures. They are experimentally tested by building a temperature sensing system integrated with a fiber loop ring-down cavity with an FBG and 1551.13 nm pulsed laser. The mathematical simulation results agree well with the experimental results, which show that the proposed system has perfect repeatability and stability under the low temperature conditions of -40 ℃. Specifically, the measurement resolution achieved is 0.05 ℃, the sensitivity of temperature is 0.0025/(μs·℃), and the sensitivity of FBG central wavelength is 0.00248/(μs·pm).