The disclosure provides a laminating device, a laminating method thereof, a flexible display module and a display device. The laminating device is configured to laminate layers of the flexible display module. The laminating device includes a stress applying component, configured to control the first laminating base platform and the second laminating base platform to laminate in a lamination direction, and control at least one of the first laminating base platform and the second laminating base platform to be deformed in a direction perpendicular to the lamination direction during lamination, such that the first laminate layer, that has been laminated, in an unfolded state is provided with a pre-applied stress, wherein a direction of the pre-applied stress is opposite to a direction of a stress borne by the first laminate layer in a folded state.