Provided is a field emission X-ray tube. The field emission X-ray tube includes a cathode electrode provided on one end of a vacuum container, including a field emission emitter; a gate electrode provided inside the vacuum container to be adjacent to the cathode electrode, including a first opening; a focusing electrode electrically connected to the gate electrode and provided on one surface of the gate electrode to be farther from the cathode electrode than the gate electrode while including a second opening with a greater width than that of the first opening; and an anode electrode provided inside the vacuum container on another end thereof in a direction where the vacuum container is extended. A height of the focusing electrode is identical to the width of the second opening, and wherein the width of the first opening is ⅓ or less of the second opening.