Due to coupling of in-wheel motor and wheel/tire, the electric wheel system is different from the tire suspension system of internal combustion engine vehicles both in the excitation source and structural dynamics. Therefore emerging dynamics issues of electric wheel arouses attention. Aiming at longitudinal vibration problem of electric wheel system in the starting condition, vector control system of permanent synchronous hub motor considering dead-time effect of the inverter is primarily built. The transient change features and ripple features of electromagnetic torque in the starting condition with successive acceleration are revealed. Based on rigid ring model and dynamic tire/road interface, coupled longitudinal-torsional vibration model of electric wheel system is then established and inherent characteristics of this model are analyzed. The vibration responses of electric wheel system are simulated by combining electromagnetic torque and vibration model. Abrupt change of driving torque will result in transient vibration of electric wheel system. In detail, the vehicle body mainly undergoes longitudinal shake in 4Hz and the unsprung mass suffers from longitudinal shock in 40Hz; the transient vibration of electric wheel system is most remarkable at zero velocity and decrease with vehicle speed. In addition torque ripple will arouse 6pth order vibration, which concentrates in 40Hz and 100Hz and becomes noticeable in certain speed range due to resonance. The longitudinal vibration of electric wheel system in the starting condition is highlighted and deserves attention. The detailed features and characteristics of this vibration are studied in this paper, which is favorable for design and analysis of in-wheel motor driven vehicle.