a back plate which includes a flat plate, first and second wedges respectively formed at both ends of the flat plate, and a plurality of third wedges formed between the first and second wedges on the flat plate, wherein an angle between a long inclined plane of the first wedge and the flat plate is an obtuse angle, an angle between a long inclined plane of the second wedge and the flat plate is an acute angle, and an angle between the inclined plane of the third wedge and the flat plate is an acute angle, wherein a side end of the first wedge that is adjacent to the third wedge and a thick side end of the third wedge are all concave to form grooves, and wherein a side end of the second wedge that is far away from the third wedge extends upward to form a second U-shaped cavity of which an opening faces towards the first wedge;

a first light source assembly disposed on the first wedge;

a second light source assembly disposed on the second wedge and comprising two second light source units and two second light guide plates, wherein one of the two second light source units is disposed in the second U-shaped cavity, the other one of the two second light source units is disposed in a groove formed by a thick side end of the adjacent third wedge, and two light guide plates are disposed on the second wedge in a butt joint way and one of the second light guide plates is provided with a support pillar thereon; and

a plurality of third light source assemblies, wherein each of the third light source assemblies is disposed on the corresponding third wedge and comprises a third light source unit disposed in the corresponding groove and a third light guide plate disposed on the third wedge adjacent to the third light source unit.