The nonlinear vortex-induced vibration of a rectangular section with an aspect ratio B/D = 6 is investigated in this study, aiming at explanation of the phenomenon of double lock-in ranges. First, numerical results by two-dimensional CFD simulation are compared with experimental results; then flow field characteristics, aerodynamic loadings and structural motion properties are presented and discussed; finally, an energy-trapping-based model for motion stability is brought forward, based on which the observed aeroelastic phenomena are discussed. The present study shows the motion-induced lock-in range is able to be explained qualitatively with the proposed principle describing the free-oscillation stability. Within the motion-induced lock-in range, the 1-DOF system can experience two motion components. The dominance of the free vibration fueled by feedback lift can dwarf or even eliminate the vortex-shedding-induced motion. Further, it is demonstrated that the phase angle between the lift and motion velocity, instead of the load amplitude, dominates the motion stability.