The present invention provides a backlight module and a liquid crystal display device. The backlight module which comprises a waveguide including a light exiting surface, and a side surface adjoining the light-exiting surface. An optical film includes a main portion disposed on the existing surface of the waveguide, and a skirt extends from a peripheral of the main portion and adjoining to the side surface of the light-exiting surface. A backboard includes a base and an upright extending from peripheral of the base, the waveguide is disposed on the base, and the upright is spaced with the skirt; and wherein a retaining device is supportably arranged between the upright and the skirt so as to press the skirt onto the side surface of the waveguide. The present invention can effectively reduce the estate of the backboard, and is beneficial to the design narrow-border of liquid crystal display device.