A field emission cathode structure includes a dielectric layer, a field emission unit, a grid electrode, and a conductive layer. The dielectric layer is positioned on the insulating substrate and defines a cavity. A field emission unit is attached on the cathode electrode and received in the cavity of the dielectric layer. The field emission unit is electrically attached to the cathode electrode. The grid electrode is located on the dielectric layer, and electrons emitted from the field emission unit emit through the grid electrode. The conductive layer is electrically attached to the grid electrode and insulated from the field emission unit. A field emission display device using the above-mentioned field emission cathode structure is also provided.