According to the real structure and work condition of a large-scale gas control valve used in recycling generating electricity project, a modal analysis modeling of fluid-structure interaction system is set up and the vibration in fluid-structure interaction is studied. The fluid force on the valve plug when the control valve at different openings is analyzed. According to the modal computing of the plug in fluid environment, the vibration frequencies of the plug are obtained. The influence on vibration frequency of fluid-structure interaction is showed, which relate to vibration modal of plug. The change of plug frequency at coupling and uncoupling condition is investigated. All of these are in favor of vibration characteristic analysis, vibration fault diagnosis of control valve system and realizing design optimization in plug dynamic characteristics. These are of great practical significance for advancing study on large-scale control valves.