A planar micro-spring based on a planar rectangular helical spring was designed for a low-gn micro inertial switch with the micro-spring in stiffness up to 0.1-1 N/m magnitude. The elastic coefficient formula of the micro-spring was derived with the Castigliano’s theory and the linear elasticity theory based on the assumption that the material property was uniform, homogeneous and isotropic, and then it was modified by the ANSYS finite element method. The micro-spring was fabricated by using the MEMS bulk micromachining process based on the multi-step ICP etching method,and the bulk of chip was about 7 mm×7 mm×0.3 mm in size. The theoretic analysis shows that the results agree well with the ANSYS simulation results, which demonstrates that the optimized formula can be directly applied to the optimization design of the micro-spring to simplify the design process. The nanoindentation test results show that the elastic coefficient of the fabricated micro-spring is about 0.554 N/m. The planar micro-spring has the advantages of small size, simple structure and easy fab-rication, and it can lay the foundation for the engineering applications of low-gn micro inertial switchs.