A 3-DOF dynamic model of vehicle shimmy was established. In this model, the external excitation of dynamic unbalance of front wheel was considered. Based on bifurcation analysis, the dynamic response of vehicle shimmy system with the excitation of dynamic unbalance was analyzed, and the characteristics of frequency components of the vibration response were systematically analyzed. The results show that the characteristics of shimmy with the external excitation of dynamic unbalance is quite different from self-excited vibration, so it should be considered in the modeling process of shimmy system. With increment of the excitation of dynamic unbalance, the vibration experiences the variations from domination of self-excited vibration, which is caused by system structure itself, to forced vibration, which is caused by external excitation. As a result, the system experiences the variations from quasi-periodic motion to periodic motion under certain vehicle speed range. The correlative conclusions can provide theoretical basis for effective control of the steering wheel shimmy.