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Based on the postbuckling theory of large deflection beams, the nonlinear stiffness of a postbuckling beam is deduced and in agreement with the results of buckling experiments. Then, a novel post machined threshold accelerometer is designed, which consists of eight oblique post beams with an inertial mass in the middle to ensure its single moving direction and an electrical contact part fabricated on the bottom of the inertial mass. The threshold accelerometer is an integration of a threshold sensor and an inertial driven actuator used in airbag restraint systems. When the acceleration reaches the threshold, the beams buckle and close the threshold accelerometer, and when it gets down to be a certain value, the accelerometer opens quickly under the effect of the elastic force developed by the postbuckling beams. Compared with the design models of other threshold accelerometers with linear beam structures, the nonlinear postbuckling beams are introduced as threshold sensing elements. A number of design factors such as the air film damping and the contact force are taken into full consideration, thus establishing the dynamic equation of the accelerometer under coupled forces. The dynamical simulation for the strong nonlinear system with elliptic integrals indicates its good threshold characteristic and high contact reliability. The threshold accelerometer responds within 4 ms when it is triggered by a threshold acceleration ac = 20 g, and cuts off quickly when the cutoff acceleration is under ad = 5 g. Meanwhile, the unstable contact time is only 0.02 ms for the contact force to reach 50 mN, which guarantees the contact resistance to be less than 20 mOmega. With the results of the dynamic simulation, supported by previous buckling experiments, the accelerometer can provide accurate threshold sensing without false actuations under interferences outside, especially electromagnetic and vibration interferences, and hence their wide applications in safe-arming systems.