This invention provides a backlight device which includes at least: a photo conductive plate used for visible light to go through; a point light source, which is beside the photo conductive plate, used for producing visible light; a first diffusing top, which is beside the photo conductive plate, used for diffusing the visible light from the point light source to the lighting method of uniform line light source; a second diffusing top, which is on the bottom of the photo conductive plate, used for diffusing the visible light from the first diffusing top to the lighting method of uniform top light source; a diffusing reflect objective, which is below the photo conductive plate, used for diffusing the light and then reflecting the diffused light to the photo conductive plate.