A dual-slope light guide includes a first surface, a second surface and a curved surface. The first surface has first reflecting structures. The second surface, connected to the first surface, has second reflecting structures. An included angle between the first surface and the second surface is unequal to 180 degrees. The curved surface connects the first surface to the second surface. The first surface, the second surface and the curved surface extend from a first end of the light guide to a second end of the light guide. A light beam incident to the first end of the light guide is reflected and converted by the first reflecting structure and the second reflecting structure into linear light emitted from the curved surface. A light source module and an optical assembly for a scanner are also disclosed.