A back plate unit includes a back plate having opposite first and second sides, an optical member mounted to the back plate, and a buffer assembly disposed between the optical member and the back plate and including at least one first buffer member disposed on the first side and made of a thermoplastic material, and at least one second buffer member disposed on the second side and made of a thermoset material. The at least one first buffer member and the optical member are deformed when heated, and the at least one first buffer member is squeezed and compressed by expansion of the optical member. A backlight module of a display device is also disclosed.