Provided are an optical illumination system, and a projection device. In an example, the optical illumination system includes a DMD, a lens assembly and a RTIR assembly. The lens assembly adjusts a light beam. The RIM assembly includes a first plano-convex lens and a wedge prism. The first plano-convex lens includes a plane surface and a convex surface and is configured to refract the light beam adjusted by the lens assembly. The wedge prim includes a first side surface, a second side surface and a third side surface. The plane surface of the first plano-convex lens is glued with the first side surface of the wedge prism. The wedge prism is configured to receive and refract the light beam refracted by the first plano-convex lens through the first side surface, and the light beam refracted by the first side surface is transmitted through the second side surface. The DMD is at a light emitting side of the second side surface of the wedge prism, and configured to receive and reflect the light beam from the second side surface of the wedge prism. The reflected light beam is incident to the first side surface of the wedge prism through the second side surface of the wedge prism for total internal reflection then transmitted through the third side surface.