To realize the rational prediction of system friction torque in the design phase and improve the correctness of performance prediction for the actuator, taking the electronic mechanical brake (EMB) actuator with planetary gear and ball screw as the study object, the main source of system friction and its influence on the EMB actuator were analyzed. Firstly, the dynamic model and friction torque model were established according to the structure and load of EMB. The prediction of the friction torque model was compared with the experimental results, and the reason of the difference between the experimental and predicted results was discussed. Based on the dynamic model and friction torque model, the influence of parameters in the friction torque model on the gap closing time and maximum clamping force was studied. The energy dissipation during braking with the maximum clamping force was calculated. The proportion of the friction torque at each component and the influence of rotation rate and clamping force on these friction torques were analyzed. It was found that the frictions which influence the gap closing time come from ball screw and motor, while the frictions which affect the capability to clamp are generated by ball screw, motor and thread bearing. With the increase of the clamping force, the influence of the frictions due to ball screw and thread bearing increased significantly, while the impact of friction from motor decreased greatly. During emergency braking, the effective torque transfer ratio was 73.56%, and the energy dissipation ratio was 48.1%.