Abstract: Traction battery is the core component of the electric vehicle. To obtain longer driving ranges, conventional lithium-ion batteries with LiMn₂O₄, LiCoO₂, and LiFePO₄ cathodes were gradually replaced by LiNi_xCo_yMn_1−x−yO₂ batteries. With the increasing energy density and chemical activity of the lithium-ion traction battery, its thermal stability gradually decreases and safety hazards become increasingly serious. In recent years, thermal runaway incidents with traction batteries have occurred frequently at home and abroad, seriously disturbing the development of electric vehicles. Solving the safety problems associated with thermal runaway(TR) and thermal runaway propagation(TRP) of the lithium-ion battery is urgent. In this paper, TR and its propagation behavior, associated with a 42 A·h prismatic lithium-ion battery with a LiNi_1/3Co_1/3Mn_1/3O₂ cathode for electric vehicles, were studied under thermal abuse conditions on the cell and module levels. The results indicate that the maximum temperature approaches 920 ℃ inside the cell. The maximum temperature difference is up to 403 ℃ within the cell during TR, and the maximum temperature rise rate inside the cell is 40 ℃·s⁻¹. The TRP time within a lithium-ion battery is 8–12 s under 100% state-of-charge (SOC), and the duration of the vent is 14–18 s. The temperature characteristics of the lithium-ion battery display large differences for the TRP test and adiabaticTR test. In a propagation test, the TR initiates from a forward surface toward the failure point, whereas under the adiabatic test the TR occurs simultaneously in the cell. More than 80% of the particles vented from the cell are LiF and graphite during the adiabatic test. Approximately 90% of the heat released by the TR is used for heating the residual and venting particles of the cell. The study offers a reference guide for the safety design and mitigation strategy of TRP in lithium-ion battery modules, and accident investigations of new energy vehicles.