The vortex-induced vibration is a kind of common phenomenon of fluid-structure interaction. When the vibration of structure is substantial, it can be used to extract energy from ocean or river currents. A numerical method using fully coupling between fluid and structure was presented for simulating the vortex-induced vibration of spring mounted cylinder in uniform currents. In order to analysis the influence of mass ratio m*, damping ratio ζ, mass-damping ratio m*ζ and natural frequency fn on the energy conversion efficiency η, four groups of cases were computed. The results reveals that η is influenced by the above parameters and there is an optimum value of m*ζ where the η presents a maximum. The conclusion is considered as a reference to the design of vibration system which based on the present concept.