A backlight module arranged under a flat display includes a printed circuit board being parallel to the flat display and including a plurality of LEDs, a reflection portion being a reflection wall bent from a fringe of the LEDs to extend over the LEDs, and a guide light plate being parallel to the printed circuit board. The reflection portion includes an orientation wall parallel to the reflection wall and an outlet with a direction parallel to the printed circuit board. The guide light plate has an end connecting inside the outlet of the reflection portion, a light delivering surface adjacent to the flat display, and a reflection surface arranged at a bottom thereof.