Abstract: To investigate the vortex-induced vibration (VIV) characteristics of elastically connected tandem double cylinders, a dynamic theoretical model of the dual-cylinder system is established. The fluid-structure interaction calculation method of ANSYS Fluent and MATLAB is adopted. The model considers two-degree-of-freedom (2-DOF) motions (cross-flow and in-line) and incorporates nonlinear spring forces. The computational results of the proposed dynamic model are compared with wind tunnel experimental data, confirming the accuracy of the numerical method and theoretical model. Numerical simulations are conducted for a tandem cylinder system at Reynolds number Re = 150 with center-to-center spacing of 2D (where D is the cylinder diameter), mass ratio of 2, and reduced velocity range of 3~10. The influence of varying connecting spring stiffness on the response characteristics is systematically analyzed. The results demonstrate that elastic connection increases the onset reduced velocity of VIV in tandem cylinders. For in-line vibration, the vibration amplitude increases with flow velocity until reaching a stable value. Higher connection stiffness leads to larger amplitudes. In cross-flow, the vibration amplitude initially increases and then decreases with increasing flow velocity. Additionally, as the connection stiffness increases, the lock-in velocity range gradually widens, and the vibration amplitude also grows. Notably, the study reveals that the connection stiffness affects the phase difference between the vibration of two cylinder, consequently altering their vibration amplitudes.