A one-step ultrasonic welding approach for metal-packaged fiber Bragg grating (FBG) sensors was proposed to solve the problem of aging and creep in the field of traditional adhesive packaging. FBGs with and without polyimide coatings were bonded to the surface of an aluminum alloy substrate via one-step ultrasonic welding. The spectrum and thermal and mechanical properties of the bonded FBGs were studied. Additionally, the cross sections of the FBG sensors were analyzed by a Scanning Electron Microscope (SEM). The results reveal that the reflection spectrum of the coated FBG sensor has no obvious distortion, and the side-mode suppression ratio is higher than 10 dB. It is shown that the temperature sensitivity coefficient of the polyimide-coated FBG is 34.63 pm/°C, the strain sensitivity coefficient is 1.18 pm/με, the strain transfer efficiency is 98.5%, and the linearity reaches 0.999, the value of which is higher than that of the uncoated FBG sensor. After performing several repetitive temperature impact tests at temperatures ranging from 14.2 °C to 80 °C, it is found that the uncoated metal-packaged FBG sensor is destroyed, whereas the coated sensor maintains excellent temperature response characteristics. The SEM results show that the metal alloy bonded well to the surface of the coated and uncoated optical fibers. Furthermore, the one-step ultrasonic welding technique was confirmed to not require metallic pretreatment on the FBG surface; thus, the technique is relatively simple and requires less time to implement. The results indicate that the polyimide coating can effectively improve the reliability of metal-packaged FBG sensors for measuring temperature and strain, which would be useful in rigid-environments and for long-term sensing.