In this study, thermal runaway propagation characteristics and measures to prevent this phenomenon were analyzed by applying abnormal thermal conditions to pouch-type lithium-ion batteries. Experiments were conducted in a 1.5 m × 1.5 m × 1.5 m experimental chamber. During the experiment, pouch-type lithium-ion batteries were grouped according to capacity, quantity, and the use of fire extinguishing agents. Experiments showed that when thermal runaway occurred in a cell, it propagated to the adjacent cell after a certain period. The surface temperature of the cell where thermal runaway first occurred was above 200 ℃, and thermal runaway propagated via heat transfer to the adjacent cell. In the case of thermal runaway, when a fire extinguishing agent was applied, the propagation of thermal runaway to adjacent cells was prevented due to a cooling effect. However, at a cell capacity of 100 Ah, flame generation persisted and thermal runaway was unavoidable. To prevent thermal runaway propagation, it is necessary to select an extinguishing agent that exerts a cooling effect. The capacity and structure of the model unit should be considered when installing fire extinguishing systems.