Based on the nonlinear contact theory,a finite element model of spring clip-III fastening system used in metro lines is established. The relation between fastening’s clamping force and vertical displacement of its finger is obtained. Stress level of spring clip is analyzed under different distance of the spring clip inserting into the shoulder. The influence of radius of the shoulder end on clamping force and stress is investigated. Based on numerical results,the radius of the shoulder end is optimized. Results show that the distance of the spring clip inserting into the shoulder greatly affects the contact state between the clip and the shoulder. A local stress concentration in the clip occurs at the clip’s backend arc surface that is contacted with the shoulder end. When the shoulder end radius is changed to 8 mm,the stress value of the local stress concentration area significantly decreases.