The present invention relates to an automatic cover device for an open-top freight car. The present invention comprises: a cover part having a cover for opening and closing the top of a loading compartment of an open-top freight car; opening and closing devices respectively provided at the front and rear of the open-top freight car and to which both ends of the cover are coupled so as to support the opening and closing operations of the cover; and a driving device including a wire member for operating the opening and closing devices, an opening and closing drive part for supporting the winding and unwinding operations of the wire member, a drive motor for providing rotational force to the opening and closing drive part, and a power supply part for applying power to the drive motor. The present invention is provided even in a state, in which conventional open-top freight car facilities are remodeled or not dismantled, such that the top of the loading compartment can be opened and closed, thereby reducing installation, maintenance, and management costs for the automatic cover device.