Abstract: Nuclear energy, as a safe, clean, and efficient low-carbon energy source, has become an important option for achieving sustainable energy and power development. However, during the operation of pressurized water reactors, flow-induced vibrations can trigger fretting wear behavior in Zr-4 alloy cladding, causing damage to the contact surface and leading to the failure of nuclear fuel assemblies, potentially resulting in the leakage of nuclear fission products. To investigate the fretting wear characteristics of Zr-4 alloy cladding, a fretting wear experimental device was independently designed and constructed for studying the wear behavior of Zr-4 alloy under line contact with different displacement amplitudes. The fretting wear of Zr-4 alloy was divided into three zones based on displacement amplitude: low wear zone, accelerated wear zone, and stable wear zone. A white light interferometer was used to obtain the 3D morphology and wear quantities (wear volume and maximum wear depth) of the damaged areas, while scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to analyze the microscopic morphology and elemental distribution of the damaged areas. The microstructural characteristics of the fretting wear zones in Zr-4 alloy were studied, focusing on the changes in micro-morphology, elemental composition, and wear mechanisms. The results indicate that an increase in displacement amplitude intensifies the wear phenomena, with maximum wear depth and wear volume increasing. Particularly, in the accelerated wear zone, fretting wear to Zr-4 alloy deteriorates more rapidly. The growth rates of both maximum wear depth and wear volume reach their peak and then decrease. Throughout the fretting wear process, metal oxides such as Zr, Fe, and Cr are generated and removed from the wear zones. The Fe content in the damaged area gradually decreases and stabilizes, while the oxygen content initially increases, reaching a peak in the accelerated wear zone before beginning to decrease. In localized wear zones, the transfer and adhesion of wear debris are observed. The low wear zone is covered by a smooth tribological layer. The wear mechanisms in this zone involve adhesive wear, mild abrasive wear, and oxidative wear. In the accelerated wear zone, indentation formation and dynamic changes in the tribological layer occur, with a weakening of adhesive wear and an enhancement of abrasive wear. In the stable wear zone, the tribological layer in the center damage area peels off in flakes, accompanied by the initiation of micro-cracks. Abrasive wear becomes more severe, along with significant oxidative wear. The results show that displacement amplitude significantly affects the fretting wear mechanism of Zr-4 alloy. This study reveals the fretting wear behavior and mechanisms of Zr-4 alloy cladding, providing valuable data and theoretical support for the safe operation and life assessment of Zr-4 alloy cladding in pressurized water reactors.