A foldable display device includes a hinge, a housing shell unit and a display unit. The housing shell unit includes two housing shells connected with left and right sides of the hinge, two support plates respectively disposed on the housing shells, and a plurality of pairs of display rails defined between the housing shells and the support plates. Each housing shell has a base shell wall connected with the hinge. Each display rail has a rail groove. The display unit is securely disposed to the hinge, and is movably engaged in the rail grooves. The hinge is rotatable between an unfolded position and an outfolding position. During rotation of the hinge, the display unit is moved along the rail grooves.