Abstract: As the capacity of wind turbines continues to increase, high-flexibility towers are susceptible to oscillatory vibrations under wind loads, which compromise structural safety and fatigue life. Nonlinear Energy Sinks (NES) are effective devices for rapidly dissipating vibrational energy, offering a promising solution to mitigate these vibrations. This study investigates the use of a vertical-type Nonlinear Energy Sinks (VNES) to control the dynamic response of wind turbine towers. The research addresses the first-order bending deformation of the towers and explores the boundary relative motion induced by the coordinated motion of the VNES, as well as its vibration absorption mechanism. A coupled dynamic model of the tower and VNES, considering boundary relative motion, is established using the principle of virtual work. The steady-state response is solved using the complex -averaging method and numerical simulations. The study also optimizes device parameters and examines the influence of spring length ratio on vibration damping effectiveness. A case study of a 5 MW wind turbine is conducted, and the tower top response under four sets of measured wind loads is analyzed to validate the performance of the VNES. The results show that the VNES, which accounts for relative boundary motion, provides superior vibration damping compared to traditional cubic stiffness energy sinks (CNES).