For the lack of formula of linear range for S-shaped MEMS planar micro-spring, this paper establishes physical and mathematical model and analysis the material stress-strain angle. The formula is deduced by calculating the strain and rotation of the basic unit of micro-spring tension. Compared with the results of the tests on micro-spring which produced by UV-LIGA process, the formula results is in about 10% higher, providing theoretical guidance for the design and application of S-shaped MEMS planar micro-spring in engineering practice.