In this study, the flutter mechanism related to acoustic propagation characteristics of a wide-chord fan rotor was investigated numerically. The first bending mode and its two and three nodal diameters traveling wave patterns were considered. The unsteady disturbance induced by the blade vibration in the duct gradually changed from cut-off to cut-on mode by increasing the blade frequency. Flutter occurred in some specific frequency range. The upstream cut-on and downstream cut-off condition with the risk of flutter previously identified in other studies was also observed in this paper. A new flutter risk frequency in which the blade frequency was less than the upstream cut-on frequency was found. The results showed that the effects of flow features and acoustic propagation characteristics on aeroelasticity were independent. To explain why flutter was more likely to occur near stall, two sets of frequencies representing different acoustic propagation characteristics were selected to carry out aeroelastic simulations for different working conditions along the same speed line. When flutter occurred, the suction side always provided positive damping, and the pressure side always provided negative damping. This study analyzed the effect of different flow features, such as shock and radial migration, in detail. The phase difference between the pressure fluctuations on the pressure side and the blade velocity played a vital role in fan flutter.