To improve the dynamic characteristics of the electromechanical transmission system under internal and external excitation, this paper proposes a new self-damping planetary transmission form, a torsional vibration damper coupled with a planetary gear (TVD-PG), in which the torsional vibration damper is used to replace the conventional structure of a certain member in the planetary gear being fixed with a box. A dynamic model of a motor and the TVD-PG system for variable speed is established, with the torsional deformation of shaft and the time-varying meshing stiffness of planetary gear taken into consideration. The dynamic characteristics of the TVD-PG system under startup and stable conditions are obtained and compared with those of traditional planetary gear transmission. The results show that the TVD-PG system can quickly reduce the fluctuation of the motor electromagnetic torque during the startup process, so that the speeds of the motor and the output can rapidly and smoothly increase. At the same time, the status of the dynamic meshing force in the planetary gear is improved under startup and stable conditions. Furthermore, because of the electromechanical coupling, the influence of the internal excitation parameters of the gear system on the motor part can be clearly observed when the system is stable.