The durable strain of fiber Bragg grating (FBG) is used to improve the sensitivities of FBG temperature sensors at high temperature. When a sensor is manufactured at room temperature, its FBG is given a pre-relaxing length according to the temperature to measure. Once the temperature rises over the given value, its FBG starts to be stretched and to work with high sensitivity. The relationship between the pre-relaxing length and the working temperature is analyzed. In experiments, when the pre-relaxing lengths are 0.2 mm, 0.5 mm, 0.6 mm, the working temperatures is raised by 25 ℃, 50 ℃, 61 ℃, respectively, and the sensitivities are almost the same (675 pm/℃). The experimental results agree well with the theoretical analyses.