The lithium-ion secondary battery has a characteristic of high energy density. In case of hazard situations, the energy released from the battery may result in exploding, which venders the needs for some protective devices as a safe guard. The safety vent is a kind of anti-blast structure designed on the shell of the battery. It will cause a release of the pressure at a designated value while the battery is performed in abnormal situations where its inner pressure is building up. This study used FEM as a tool to assist the design of the safety vent, and a simple and effective design of the safety vent was obtained as a result. This study also attempts to find the parameters that influence the amount of breaking pressure for this type of safety vent. From a systematic approach, an operation window that can be applied directly to the safety vent design was devised from this study.