A numerical investigation on vibration characteristics of flow around a circular cylinder was carried out. This study focused on mechanic properties， such as amplitude and spectrum， of vortex-induced vibration with both single degree of freedom （DOF） and two DOFs， and the applicable conditions of these two models were also discussed. Results indicate that for single DOF vortex-induced vibration， with the increment of reduced velocity， the time-average value of drag coefficient increases firstly and then decreases， whereas the amplitude of lift coefficient trends to increase at first， then decrease， and then increase again. Moreover， both of them fluctuate when they enter or leave the locking interval. For two DOFs vortex-induced vibration， the dimensionless amplitudes of both the cross-flow and in-line vibration firstly increase and then decrease with the increase of Reynolds number. The in-line vibration cannot be neglected for low mass ratio， whereas it can be neglected for high mass ratio.