Abstract: Vortex-induced vibrations of an elastically mounted circular cylinder will be altered through influencing the development of the boundary layer of the surface and the vortex shedding by the added smaller cylinders. The excitation or suppression of vortex-induced vibrations can be obtained by changing the arrangement and number of the small cylinders. In the former, more fluid energy can be transformed into mechanical energy or electricity while the latter can be applied to protect the structures. Numerical simulations of a transversely vibrating cylinder with two small cylinders behind were conducted, where the Reynolds number is 100, based on the main cylinder, the mass ratio is 2.0 and the reduced velocity is 3~11. The diameter ratio between the small and the main cylinder is 0.125 and the gap ratio is 0.125. Results indicate that the small cylinders can change the vibration of the main cylinder significantly in the simulated control angle range of 30°~90°. When the control angle is small (30°), the small cylinder suppresses the vibration of the main cylinder. The response can be divided into two branches, i.e. VIV-and galloping-branch, at the control angle of 45°~60°. The vibration amplitude increases monotonically with the increasing reduced velocity in the galloping branch. When the control angle is large (75°~90°), the promotion from the small cylinder decreases with the increase of the control angle. Furtherly, mechanisms of the small cylinders are explained by combining vortex shedding and pressure distribution around the cylinder of different instants in one period. Analysis of the energy coefficient indicates that the energy transferred from the fluid to the main cylinder decreases with the reduced velocity, which is caused by the variation of vortex structures.