The low-g inertial switch is a kind of inertial device sensitive to the linear acceleration and achieves the switch closure when subjects to the applied acceleration。Due to its micro size,the switch is designed as a structure with the micro mass and low stiffness. To achieve a low stiffness beam in a micro plane,the planar spiral beam based on Archimedes′spiral is designed. The inertial switch is composed of a substrate,a frame structure,a cover plate and an inertial sensing element containing a sensor mass and a spiral beam.After analysing the inertial sensing element with an ANSYS and fabricating with an UV-LIGA,the laboratory centrifuge tests are performed on the low-g inertial switch to measure the on-state threshold. The result of the test is 21.27g,which is the average of 21.22,21.39,and 21.20g. It is shown that the low-g mechanical inertial switch has a closed precision of 0.5g and good repeatability in multiple tests.