Abstract: The wear on the local area of the steel wires' surface is due to the torsional fretting in the working process of the stranded-wire helical spring. Two mathematical models to calculate the normal contact force among the wires and the angular displacement respectively were established at first when the stranded spring was subject to impact load. With the experimental parameters obtained from the models, the torsional fretting test using a cylinder-cylinder contact mode, which authentically stimulated the torsional fretting among the wires in the working process of the spring, was realized successfully on a newly developed fretting tester. The running behavior and the failure mechanism of the spring wires under the torsional fretting conditions were systematically researched. It is learned that the behavior of torsional fretting strongly depended upon the normal contact force, the angular displacement amplitude and the cycles. There are three basic types of T-θ curves during the process of torsional fretting, i e. parallelogram, elliptical and linear T-θ curves. In the partial slip regime, the damage was mild and its main characteristic was abrasive wear and the mild oxidative wear, whereas the wear mechanisms were abrasive wear, oxidative wear and the delamination accompanied by the obvious plastic deformation in the mixed fretting and slip regimes.