The stress distribution of an S-shaped micro spring fabricated by the micro-electro-mechanical-system (MEMS) technology was analyzed by the finite-element method (FEM) using ANSYS software, which showed that the stress concentration is located in the inner corner of the turning round. To reduce the maximum stress but not change the spring coefficient, an optimization S-shaped micro spring with the slope cross section was designed. The width of one end of the turning round is increased from the original 80μm to 100μm, while the other is decreased from 80μm to 21.5μm. The spring coefficient formula of the optimization S-shaped micro spring was calculated out by the Castigliano second law, and the difference between the formula and the FEM is 2.7%. At the same time the FEM simulation shows that the maximum stress of the optimization S-shaped micro spring can be reduced by 32.7% while the spring coefficient is the same comparing with the primary S-shaped micro spring, which shows that the mechanical performance of the optimization S-shaped micro spring is better than that of the primary S-shaped micro spring.