Abstract: In order to analyze the reasons for type III rail fastening clip fracture in metro lines, a detailed finite element model of the rail fastening system was established to study the effect of the clip’s different installation states and the vertical displacement of its finger on its clamping force and stresses. Results show that the when the length of the center leg of the clip inserting into the shoulder is between 69 mm and 76 mm, it has little effect on the clamping force, but a large effect on the maximum equivalent stress in the clip. As the distance increases, the contact state between the clip and the shoulder becomes multi-point contact instead of line contact, and localized extrusion pressure occurs between the rear arch of the clip and the shoulder end, which results in a stress concentration in the clip. After repeated passage of trains, crack initiation can easily occur in the clip’s stress concentration area, and its crack propagation may finally accelerate its fatigue fracture. The numerical results agree well with the fracture position of a majority of clips in metro lines. In track operation and maintenance, the distance between the inner arc of the rear arch and the shoulder end should be between 8 mm to 10 mm.