Vortex-Induced Vibration (VIV) is a kind of high-energy phenomena and can be used to harvest energy from ocean/river currents. In this paper, a spring-mounted circular cylinder in VIV was numerically investigated with 2-dimensional simulations to examine the effects of submergence depth on the energy conversion. The flow speed changes from 0.2 m/s to 1.3 m/s (1.61 × 104<Reynolds number<1.05 × 105), covering three different VIV branches (i.e. initial branch, upper branch and lower branch). The submergence depth changes from 0.1 m to 0.5 m. Results indicate that proximity to the free surface significantly affects the VIV responses. As the submergence depth decreases from 0.5 m to 0.1 m, the VIV amplitude is gradually suppressed resulting in a similar drop in energy conversion. The maximum energy conversion efficiency (34.7%) is achieved in the VIV upper branch with the submergence depth S = 0.5 m. When the submergence depth reaches up to the critical submergence depth S = 0.5 m, the VIV amplitude and converted power no longer increase, which means the effects of submergence depth or the disturbance from the free surface can be ignored. It is also found that when the flow speed exceeds 1.2 m/s (Reynolds number>9.7 × 104), the oscillation amplitude of the cylinder decreases to zero. In this case, the oscillation response doesn’t belong to VIV.