Backlight module and diffusing plate thereof, and planar display using the same

The present invention discloses a backlight module, which comprises a metal back frame, a light source, a diffusion plate and at least one optical blank sheet. The metal back frame is provided with a top opening and a bottom surface corresponding to the opening; the light source is arranged on the bottom surface of the metal back frame; the diffusion plate comprises a diffusion plate body, a fixing part and a protruding part, and is provided with an upper surface and a periphery; the fixing part is arranged at the periphery of the diffusion plate body; the protruding part protrudes out of the upper surface of the diffusion plate body and forms a holding space together with the upper surface; the fixing part is used for fixing the diffusion plate on the metal back frame; and the optical blank sheet is positioned in the holding space. The present invention adopts the design with the diffusion plate clamped and connected with the metal back frame through the fixing part, and thus achieving the effects of bearing