A flexible display device manufacturing method includes preparing a substrate assembly in which a flexible substrate is formed on a carrier substrate; piling up a plurality of the prepared substrate assemblies in a heating furnace in multi-stages; performing heat treatment by providing hot blast onto each of the piled substrate assemblies in a horizontal direction; forming a display unit on the flexible substrate of the heat-treated substrate assembly; and separating the flexible substrate and the carrier substrate from each other. According to the above-described manufacturing method, since warpage of a flexible substrate after a carrier substrate and the flexible substrate are separated from each other may be suppressed, a subsequent process may be appropriately performed, productivity may be improved, and damage of products caused while the flexible substrate is handled may be reduced.