The aim of this paper is to study the characteristics of torsional vibration of a rotor with unbalance by numerical simulation. It can be concluded that synchronous torsional vibration accompanying with small higher harmonic components are excited, except when the speed of rotation is near, or equal to, half the natural frequency of torsional vibration, the bisynchronous component is much more remarkable than other harmonic components including the degraded synchronous one, and torsional vibration of the shaft can result in lateral vibration with bisynchronous frequency. Especially, when the rotating frequency is near the natural torsional frequency, where torsional vibration is strongest, the bisynchronous vibration becomes rather strong.