Abstract: Starting from the operating principle of interlock device on the ejection seat and the working order of its agencies, we analyzed the reasons why ignition institution worked in advance, and established the finite element analysis model of the key component-lock arm in the interlock device. Through the simulation analysis, we found clearly the maximum deformation site of the lock arm, the risk strength parts of the lock arm, and proposed the modification program of the lock arm in the structure size, and finished the deformation and stress analysis of the modified lock arm. The results showed that the end maximum deformation of the lock arm was reduced, and the stress intensity of the weak parts was significantly lower than before, which verified the effectiveness of the modifi-cation program. The study provides a reference for the optimal design of key components on the interlock device.