Lithium-ion battery is one of the most versatile energy storage technologies today, and the reliability and safety of lithium battery have always been the target pursued by the industry all the time, so it is particularly important to accurately monitor the safety status of the battery. Actually, the ultimate cause of all lithium battery safety problems lies in the thermal runaway inside the lithium battery. In order to overcome the current problems of temperature measurement systems, such as low accuracy and insufficient stability for long-time operation at relatively high temperature, a temperature monitoring system of quasi-distributed lithium battery based on double clad Fiber Bragg Grating (FBG) is proposed in this work. After the monitoring of the temperature field and bulge deformation of 18650 lithium battery pack by building 4 channels and 16 double clad FBG points to monitor the temperature field and bulge deformation of 18650 lithium battery pack, the results show that the points with abnormal temperature rise caused by short circuit and other problems can be accurately determined under the temperature of 0–450 ℃, with the corresponding temperature sensitivity of 10 pm/℃, and the resolution of 0.1 ℃. The double clad FBG attached to the surface of the lithium battery shell can also monitor the bulge deformation on the surface of the battery shell, and its longitudinal pressure modification sensitivity is up to 142 pm/N. The temperature field monitoring system of quasi-distributed lithium battery pack based on double clad FBG in this paper can not only ensure high-precision temperature and deformation measurement, but also have good stability and anti-interference ability, which shows that the research work in this paper is expected to provide a reliable theoretical and experimental basis for the safety monitoring and use of lithium battery pack.