Micro-Electro-Mechanical System(MEMS) inertial switch has broad application prospects in the field of fuze, but there are few low-g MEMS inertial switches that can be used in the working environment of fuze. A design of MEMS inertial switch with low closing threshold is proposed for the fuse of ammunition with low launch overload, to meet the functional requirements of the fuze to identify the low-g value impact signal, and to realize the function of stable connecting the fuze power supply. At the same time, the designed MEMS inertial switch has the ability to resist high overload in the fuze's working environment and improve the reliability of the fuze. The design requirements are put forward according to the characteristics of the trigger signal and the working environment of the fuze. The oscillation model of the inertial switch is theoretically analyzed. The influence law of the structural size of the inertial switch on the switch performance is clarified. The overall design scheme of the switch is given, and the design results are simulated and verified by ANSYS finite element analysis software. The simulation results show that the designed inertial switch can monitor the impact signal with amplitude of 20g and pulse width of 1 ms, the response time is less than 1 ms, the electrode contact time is longer than 30 μs and anti-overload performance reaches 12 000g, meeting the proposed performance index requirements.