A flexible display device substrate and a preparation method thereof are provided. The flexible display device substrate includes a first flexible substrate, a toughening layer, a second flexible substrate, and an interweaved structure layer. The second flexible substrate is disposed on the first flexible substrate. The interweaving structure layer is disposed between the first flexible substrate and the second flexible substrate, wherein the interweaving structure layer is formed by a part of the first flexible substrate and a part of the second flexible substrate penetrating and interweaving with each other. The toughening layer is disposed at intervals between the first flexible substrate and the second flexible substrate, and the toughening layer and the interweaved structure layer are disposed alternately.