In order to investigate the action mechanism of the high-speed train yaw damper and provide theoretical guidance for selecting the optimal damper parameters. The frequency-dependent characteristics of the damper and the optimal energy dissipation conditions are analyzed. The yaw damper parameters are optimized with multi-objective based on two types of typical high-speed train lateral dynamic models. The linear system stability and modal energy of the vehicle are comparatively analyzed to summarize the action mechanism of yaw damper. The conclusions are as follows：Not only the damping of yaw damper dissipates the vehicle hunting energy，but also the stiffness characteristic has a more significant impact on the vehicle lateral stability. The stiffness of the yaw damper should be increased with the increasing of hunting frequency. The mechanism of yaw damper equivalent stiffness characteristic is explained with the modal energy distribution and implicative effect between the car-body and the bogie. Besides，the matching of the yaw damper series stiffness and damping is realized according to the optimal energy dissipation theory. The design ideas and the structure scheme of frequency-dependent stiffness yaw damper are proposed，and the optimization of frequency-dependent stiffness curves as well as the vehicle system stability analysis is carried out. It is shown that the frequency-dependent stiffness yaw damper can significantly improve the vehicle lateral stability under different wheel-rail contact conditions，reducing the risk of low-frequency lateral sway and high-frequency shaking of high-speed train，which plays a positive role in the realization of vehicle adaptive stability in different wear stages of the wheel tread.