A cover unit for a USB flash disk, wherein a seat hole is provided at the middle position of a seat, an engaging structure with an elastic element is pivotally provided in the cover unit to provide restoring force, an engaging plate of the engaging structure can be extended into the opening of the USB connector, and by using protruding blocks provided on the engaging plate in matching with directional holes on the USB connector, the holes are covered by the blocks to seal the USB connector; meantime when a stopping plate on the engaging plate is pressed, engaging of the protruding blocks with the directional holes can be relieved, thereby the whole cover unit can be removed.