A light processing structure for a digital light processing (DLP) projection device is disclosed. The DLP projection device comprises a plurality of digital micro-mirror devices (DMDs). The optical processing structure comprises a color separation mechanism, a reflecting mechanism and a color combination mechanism, wherein the color separation mechanism utilizes dichroic mirror(s) and a color wheel for splitting light beams into individual colors. The reflecting mechanism includes a plurality of TIR prisms corresponding to the DMDs, respectively. The color combination mechanism utilizes a color-combining prism assembly in which two triangular prisms are assembled with each other. The mentioned mechanism uses a shorter back focal length for the projection lens, thereby reducing the light processing structure and enhancing product quality.