The optical fiber Bragg grating (FBG) sensor has great potential to be used for measuring temperature, strain, or even detecting fracture in metal materials. Embedding FBG sensor into metal materials is difficult and changing. Metal coating is one of the most efficient methods to protect FBG. Uniform metal coating can be applied in FBG sensor using electroless plating. The temperature changes induce thermal stress between the metal coating and the FBG due to their different thermal expansion coefficients. For the nickel-clad FBG, the thermal stress was analyzed and the math model about the coating thickness influencing temperature sensitivity was built. Temperature increase and decrease sensing experiment of the nickel-clad FBG shows, for increasing temperature and decreasing temperature, the errors between the practical temperature sensitivity coefficient and the model coefficient are 6.22% and 6.75% respectively. Compared with the naked FBG, the temperature sensitivity of the nickel-clad FBG is increased to two times and more. The results show the model is feasible to explain the temperature sensitivity enhancing mechanism.