The invention relates to a car door system which comprises a salver fixed on the vehicle, a mechanical connected transmission device, a door transmission device, a door locking device and a door leaf device. It is characterized in that said transmission device also comprises a planet external slide device, a belt wheel connection plate, a tension plate, a flexible belt connection plate, a slide axle, a solar wheel and a vertical external device. Wherein, said door transmission device comprises a door leaf transmission device; said door locking device comprises a locking hook, a hasp and a hook connection plate. The invention utilizes the motor to directly drive the planet external slide device, with large transmission ratio, two-stage transmission ratio, lower output torque, and less power consumption, while avoiding the interference between door leaf and door frame in switching the door. It has better sealing property and lower cost.