An optical diffusion plate and a light source module are provided. The optical diffusion plate includes a light incident surface, a light emitting surface, at least one pattern region, and a reflection layer. The pattern region is located on at least one of the light incident surface and the light emitting surface. The reflection layer covers a portion of the pattern region. The pattern region includes a central region located at a central position, a plurality of first circular regions, and at least one second circular region. The central region is surrounded by the first circular regions and the second circular region alternatively, and perimeter of the central region is adjacent to one of the first circular regions. Coverage rates of the reflection layer at the second circular region and the central region are higher than coverage rates of the reflection layer at the first circular regions.