The invention discloses a diffusion plate for use in a backlight module with a low optical path distance (OD), which can be assembled on a backlight module with a plurality of light-emitting diodes (LEDs) as the light source below. Different diffusion particle additives are added to the surface layers and the main layer of the diffusion plate, and then extrude it with foaming extrusion technology. By making different combinations of the refractive index and the amount of the original resin materials, microbubbles, and diffusion particle additives of the surface layers and the main layer, the light refractive index of the upper and lower surface layers is substantially greater than which of the main layer. Such that, the light emitted by the light source below can be diffused more effectively, and thereby achieving a better shading effect of MURA, so as to produce a uniform surface light source.