The invention provides a backboard, a backboard assembly, a backlight module and a display module. The backboard comprises a bottom plate and side walls, the side walls are perpendicularly connected with the periphery of the bottom plate to form an accommodation cavity used for accommodating a light guiding plate, and openings which face towards the accommodation cavity are formed in the side walls; the widths of the parts, close to the center of the bottom plate, of the openings are smaller than that of the parts, away from the center of the bottom plate, of the openings. The backboard assembly comprises the backboard which is provided by any of the embodiment of the invention and comprises hooks arranged in the openings. The backlight module comprises the backboard assembly provided by any of the embodiment of the invention and diaphragms. The display module comprises a display panel and the backlight module provided by any of the embodiment of the invention. According to the provided backboard, backboard assembly, backlight module and display module, wrinkling caused by the stretching failure of the expansion of the diaphragms when the diaphragms are heated can be prevented.