A plug security structure includes a plug, a cable, a jacket, a movable cap and a key. By means of a middle block, the movable cap is slidably insertable into sliding grooves in between two upright sidewalls of a front bracket of the jacket to stop the front stop block and to have outer ribs of the elastic retaining members of the movable cap be stopped at rear end edges of the sliding grooves in the upright sidewalls of the front bracket, preventing the latch from downward displacement to unlock the plug from the mating jack and assuring a high level of network data transmission safeness. The cap has a raised portion for friction engagement with the bottom surface of the stop wall of the front bracket of the jacket, preventing vibration or displacement of the movable cap and assuring positive operation of the latch of the plug.