Metal coating can provide fiber Bragg grating (FBG) effective protection and improve the FBG sensor’s performance in harsh environments. However, the metalizing can impact the optical properties of the FBG. As a result, the sensor accuracy of FBG will lower. The reason for the effect is the stress produced in the metalizing. In this study, we coated the FBG by the electroless-electroplating method. The common changes of the FBG’s spectrum after the metalizing were apparent side-lobes and reduced peak loss, and the hysteresis error for temperature sensor was more than 3.5°C. To reduce the deformation of the FBG’s spectrum in the metalizing, we analyzed the origin of stress, calculated the FBG’s refractive index perturbation due to the thermal stress, and simulated the spectrums of FBG under different stress distributions. On the basis of this, we developed a stress control technology. After using the technology, the optical properties of the metal-coated fiber Bragg gratings have been retained, and their hysteresis errors for temperature sensors were less than 1.5°C. The results showed good repeatability. The study can be used for regulating metalizing processes, and for providing valuable information for metalizing other optical fiber devices.