The present disclosure relates to the field of display technology, and provides a back light unit, a fabricating method thereof, and a display device in embodiments. The back light unit includes a composite layer. The composite layer includes a light guide layer and a first substrate opposite to each other, and a plurality of first microprisms and a plurality of second microprisms between the light guide layer and the first substrate. Specifically, the light guide layer includes a light exit surface facing the first substrate, and the plurality of first microprisms are on the light exit surface of the light guide layer. The plurality of second microprisms are on a surface of the first substrate facing the light guide layer. Moreover, the first microprisms are fixedly connected to the second microprisms.