Embodiments of the present invention disclose an engagement structure, a crystal backlight module and a display device. The engagement structure is used for fixing an arrangement of the back cover and the mold frame of a display device. The engagement structure comprises a first engaging part provided at the intersection of a first side wall and a second side wall of the back cover, a second engaging part provided on the second side wall of the back cover, and a third engaging part provided on the mold frame corresponding to the first engaging part, wherein the first engaging part and the third engaging part are engage with each another, the second engaging part and a corresponding side surface of the mold frame are pressed against each other, when the back cover and the mold frame are assembled.