Abstract: Based on the theoretical formula of fluid force acting on a lateral vibrating cylinder immersed in unbounded fluids,expressions for dimensionless dynamically additive mass and damping are obtained by inductive definition and non-dimensionalization.The dynamic characteristics of a lateral vibrating cylinder in unbounded fluids are studied by analyzing the influences of several factors on the dynamically additive mass and damping.It is discovered that the dynamic characteristics of the cylinder are represented by the dynamically additive mass and damping,but not the dynamically additive stiffness.The dynamically additive mass and damping both consist of two portions,namely stagnation,which represents the nature of laminar flow,and turbulence,which represents the nature of turbulent flow.Their contributions depend on the flow state.The stagnation portion dominates in stagnate flow,and the turbulence portion dominates in turbulent flow.Further,the dynamic characteristics of the cylinder are significantly influenced by fluid density and viscosity,cylinder radius,and the cylinder's vibrating frequency.