The invention discloses a foldable flat plate device at the back part of a seat back, comprising a flat plate, a support rod and a handle, wherein the flat plate is installed at the back part of the seat back through a connecting piece, one end of the support rod is connected with the bottom surface of the flat plate, a roller is installed at the other end of the support rod, and the handle is installed on the lower surface of the flat plate and is provided with a pull rod. A passenger can conveniently fold and unfold the flat plate by operating the handle and the pull rod, and the flat platecan not vibrate or automatically detach from the original position because of bumping; when being unfolded, the flat plate can be used for placing articles and also be supplied for the passenger resting by bending over; and when the flat plate is folded, the space can be saved, and the handle can be used as a handle arranged behind a seat of an ordinary passenger car. The invention has the advantages of ingenious conception, simple structure, convenience of operation, safety and reliability, can satisfy the needs of people for placing the articles and resting on travel and bring people life more convenience and comfort.