The secondary lithium ion battery with its high specific energy, high theoretical capacity and extended cycle life is a promising choice as the power source for electric vehicles. In order to provide sufficient capacity, the lithium ion battery pack generally consists of a few battery modules, and the battery module includes many individual cells that are connected in parallel. While the energy and power density of lithium ion batteries are steadily improving, the thermal safety of battery module remains to be a critical challenge. In the charge and discharge process, if the cells cannot dissipate the internal heat into ambient environment completely, the heat accumulation will occur. Then the high temperature of the triggering cell will probably cause catastrophic thermal runaway. This work focuses on the simulation of 11-parallel modules which were made of commercial 18650 cylindrical cells in different state of charge (SOC) when the center cell was triggered to thermal runaway. And also the function of the top venting has been analyzed. The study shows that a proper venting valve in the cell is an effective method to restrain or postpone the thermal runaway propagation.