Based on CFD method, study about the effect of mass ratio on vortex-induced vibration (VIV) concerning rigid cylinder has been carried out. Numerical simulations of 2-degree freedom fluid-structure interaction relating rigid cylinder with mass ratios of 1, 2.4 and 7 have been performed. The relationship between normalized amplitude and reduced velocity about two mass ratios has been presented, as well as the eight-circle cylindrical trajectory, beat and phase switch phenomena. The preliminary research shows that the ‘lock-in’ region relating mass ratio of 1 and 2.4 widens than that relating mass ratio of 7. At the same time, the cross-flow maximum amplitude is larger than the latter, leading to the occurrence of obviously enhanced VIV. In addition, through analyzing the effect of mass ratio on coupling oscillation due to in-line and cross-flow vibrations, it can be found that the effect of in-line vibration on the final vibration form could not be ignored, when the mass ratio is relatively lower, e.g. 1 and 2.4 in this paper. On the other hand, the influence is very small, in the case of higher mass ratio, e.g. 7 in this paper. Finally, ‘2S’ and ‘2P’ vortex shedding modes have also been found in the wake.