A simple but efficient color mixing system for micro-projection display is presented in this paper, which has employed off-axis arranged LEDs and specifically designed dichroic mirrors in order to achieve higher optical efficiency and a better space utilization ratio compared with previous systems. The design method of the dual-dichroic-mirror (DDM) system is investigated both analytically and numerically. The design results are consistent with the theoretical calculation, and the advantages of the proposed DDM system are obvious. Compared with traditional systems that are commonly used in micro-projectors, the DDM system has indicated a much higher collection efficiency (up to 93.36%) with a significantly reduced size of 1.47 cc. Equipped with the DDM system, the designed micro-projector has realized an energy utilization ratio of 76.92% and an irradiance uniformity of 96.59% with an ultra-compact volume of 28.8 mm×28.1 mm×10 mm, which is competent for embedded projection display. It is believed that such a design has good prospects in commercialized production in the future.