Here, a distributed optical fiber temperature sensor is embedded in a lithium-ion battery to realize real-time distributed in-situ monitoring of the temperature field in the battery and the evaluating and forewarning of its operating health. The distribution state and evolution of the temperature field in lithium-ion batteries under different operating environments are analyzed theoretically according to the structure design model of the batteries. Accordingly, the characteristic temperature points (positive taps, negative taps, and center point) are selected to optimize the layout location of sensors for the accurate measurement of the temperature field and the optimization of the cost performance. Hence, the number of sensors used, the difficulty of the layout process and the cost of demodulation equipment can be reduced. Distributed cascaded fiber Bragg grating temperature sensors are employed in the experiment. The experimental results show that the temperature of each characteristic point slowly increases with the ambient temperature, while the central temperature point exhibits a rapid temperature increase, which are consistent with the theoretical results. The proposed method provides a technical reference and an implementation scheme for the in-situ monitoring of the health status of the integrated components of large-scale lithium-ion batteries in the future.