A theoretical method of in-fiber Bragg grating temperature sensing has been proposed. Based on a temperature sensing model of in-fiber Bragg gratings, it was deduced that the analytical formulae of first-order and second-order and effective linear sensitivity coefficients of in-fiber Bragg grating temperature sensing. The theoretical values of each sensitivity coefficient was calculated. Conic multinomial has been obtained by experiment. The results of theory and experiment were compared and analyzed. Furthermore, the influence of quartzose mechanical parameters on temperature sensing characters of in-fiber Bragg gratings and the application range of linear and nonlinear relations of reflected wavelength versus temperature were also discussed.