The present invention provides a backlight module and a liquid crystal display device using the backlight module. The backlight module includes: a backplane (2), a light guide plate (4) arranged in the backplane (2), a backlight source (6) arranged in the backplane (2), and an optic film assembly (8) arranged on the light guide plate (4). The backplane (2) comprises a bottom plate (22) and a plurality of side plates (24) perpendicularly connected to the bottom plate (22). The bottom plate (22) has a surface facing the light guide plate (4) and defining a curved surface (222) and the curved surface (222) comprises a curved reflection surface (224) formed thereon. The backlight module and the liquid crystal display device using the backlight module according to the present invention have simple structure and include a backplane having a bottom plate that has a surface facing the light guide plate and including a curved reflection surface having excellent reflectivity formed thereon to replace a conventionally used reflector plate so as to reduce the thickness of the liquid crystal display device, reduce the cost of manufacturing, and facilitate achievement of thinning and also preventing the light guide plate from damage caused by friction between the reflector plate and the light guide plate and thus extending the life span of the light guide plate.