Abstract: As the friction torque and transmission efficiency of planetary roller screw changed with the rotation speed of screw, the dynamic model of planetary roller screw was established based on the Lagrange method considering the frictional resistance due to the lubricating oil viscosity, the roller spin sliding and the differential sliding of the thread contact point. And the friction torque and transmission efficiency were calculated. The influences of load and angle acceleration of screw on the dynamic friction torque, transient transmission efficiency and overall transmission efficiency of the mechanism were analyzed. The results show that the friction torque of the planetary roller screw increased with the relative sliding speed of the thread contact point, and the viscous of the lubricating oil and spin sliding in the unsteady state was the main factor inducing friction torque. The overall transmission efficiency of planetary roller screw increased when it operated with large load and the short acceleration time of the screw.