According to the temperature sensing model of the fiber Bragg grating (FBG), a theoretical method of temperature sensing capacity of FBG is proposed. Based on the temperature sensing model of FBG, a temperature sensing experiment was completed at liquid nitrogen temperature (2196uC). The theoretical and experimental results were compared and analyzed, which show that at liquid nitrogen temperature or in a large-scope temperature sensing, the relationship between thermal variation DT and relative shift of reflected center wavelength DlB/lB of FBG is nonlinear and conic multinomial.