A new piston constant friction damper is designed by the energy principle and hysteresis of friction damper in this paper. The friction damper is composed of steel outer tube, steel shaft, bronze friction wedge, steel pressure wedge, coil spring, and cap, and it can be adjusted the size of friction conveniently. The relationship between friction from sliding force and spring pre-stress are guided. Hysteresis tests under a variety of displacement excitation are conducted. By controlling the displacement amplitude and changing the size of sliding force and the vibration frequency, the restoring force of friction damper were measured, the hysteresis of friction damper at different excitation frequencies and sliding force were obtained. The hysteresis curve of the piston constant friction damper is a rectangle approximately; this is the same with theoretical analysis of friction damper.