Dynamics of robot wrist is concerned with the relations between the forces acting on the robot wrist mechanism and the accelerations it produces. Many valuable contributions come forth in recent years, as a suitable dynamics model for a robot wrist is very crucial for analysing its behaviour, on-line control of motions and forces, trajectory design and optimization, and design of robot mechanisms. However, current researches always focus on full-actuated robot wrists. The proposed underactuated robot wrist is a novel mechanism with fewer actuators than the degrees of freedom. As the existing dynamics models for the full-actuated robot wrists are not suitable for the underactuated robot wrist anymore, the dynamics model of the underactuated robot wrist becomes an important issue. This article is devoted to a model for dynamics analysis of a novel underactuated robot wrist. After this underactuated robot wrist is introduced, the dynamics analysis of the underactuated robot wrist with numerical simulation based on virtual prototyping is proposed in detail. The peak values of servo motor in the joint motion unit are also estimated. The dynamics analysis of a novel underactuated robot wrist is used to demonstrate the proposed method.