In this study, vortex-induced vibration (VIV) of a circular cylinder at the low Reynolds number of 200 is simulated by a transient coupled fluid-structure interaction numerical model. The transient coupling between the fluid and the structure is updated by on-line transmission between fluid dynamic loads and structure response data. The structural vibration of the cylinder influences the flow around it, and the change of fluid flow in turn influences the response of the cylinder. The boundary layer near the cylinder is updated by dynamic mesh technology with grid updating at each time step. This method successfully simulates the vortex generation and the real-time flow field of the cylinder. Considering VIV with low reduced damping parameters, the trend of the lift coefficient, the drag coefficient and the displacement of cylinder are analyzed under different oscillating frequencies of the cylinder. The frequency ratio α is a very important parameter, when α is small (. α=. 0.5), the amplitude of lift coefficient of an elastic cylinder is large and the response of cylinder is weak. With an increase in α, the lateral displacement of the cylinder increases, but the amplitude of lift coefficient decreases. The amplitude of the lift coefficient reaches its minimum value when α is between 0.9 and 1.2. After that, the amplitude of the lift coefficient begins to increase. The typical nonlinear phenomena of locked-in, beat and phase-switch can be captured successfully. The evolution of vortex shedding from the cylinder with varied α is discussed. The trajectory of the two degrees of freedom (2 DOF) case at different α is also discussed; all appear to have a "Fig. 8" shape. A fast Fourier transformation technique is used to obtain the frequency characteristics of the cylindrical structure vibration. Using the 2 DOF cylinder model in place of the one degree of freedom (1 DOF) cylinder model presents several advantages in simulating the nonlinear characteristics of cylindrical structures including the capacity to model the crosswise vibration generated by in-line vibration.