Thermal runaway occasionally happens in batteries. A single battery, after thermal runaway, will release heat and transfer it to neighboring batteries, leading to thermal runaway of battery packs. Thus, it is necessary to explore the diffusion law of thermal runaway in battery modules. Heating is by far the most common way to trigger thermal runaway propagation of battery modules. In this paper, experiments and simulations were conducted to explore the influence of different heat insulation thicknesses on the thermal propagation of lithium iron phosphate batteries, and the result shows that the best thickness between adjacent batteries is 2 mm. For complex modules, the simulation analysis shows that when the spacing between adjacent modules in the battery pack was 10 mm and thermal runaway occurred on one side of the battery pack, it did not occur on the other side for a certain period of time. Therefore, the recommended spacing between modules in the battery pack is 10 mm. This lays a foundation for the safe design of battery modules.