A backlight module has two lighting sides installed inside a casing. Two sides of the casing are installed with a main display screen and a sub-display screen. The backlight module comprises at least one light source; a light guile plate for guiding light from the light source to the main display screen and the sub-display screen. A lower diffusion sheet, a first light enhancing sheet and preferably, an upper diffusion sheet are installed between the main display screen and the light guide plate. A reflective sheet, a second light enhancing sheet and preferably, an upper diffusion sheet are installed between the sub-display screen and the light guide plate. Part of light from the sub-display screen is reflected to the main display screen.