A dynamic model of the planetary roller screw mechanism (PRSM) is proposed in this paper, which accounts for the axial components of elastic deformation threads. The bodies of the screw, roller and nut are discretized as spring-mass systems by using equivalent finite element method. Using Newton’s second law, the motion equations of the PRSM are derived. The load distribution among threads, the screw driven torque and the contact forces acting on the pins and spur gears are obtained from the proposed dynamic model. The sinusoidal response of the PRSM is simulated and the influence of the screw rotational velocity on the dynamic characteristics of the PRSM is analyzed. The results show that the amplitudes of the forces and torques are increased with the increase of the amplitudes of the screw rotational velocity. The influence of the screw rotational velocity on the screw driven torque and contact forces of the pin and thread teeth is much greater than that on the contact forces acting on the spur gears of the roller. The contact forces acting on the thread teeth change along with the screw rotational velocity, although the external force of the nut is a constant.