With the rapid development of optical measurement and remote sensing, the demand for weight, volume and environmental adaptability in folding optical systems are continuously increasing. Metal mirrors based on additive manufacturing technology are gradually gaining the attention and research of scholars at home and abroad for their easy to realize optimum design, rapid manufacturing process and high processing performance. Compared to conventional metal mirrors, additively manufacturing metal mirrors strengthen the stiffness of the mirror and achieve a higher degree of weight reduction simultaneously. Furthermore, additively manufacturing mirrors can meet the environmental adaptability and rapidity requirements of optical systems. This paper first discusses the evaluation indicators of metal mirrors. Second, the development status and technical parameters of metal mirrors based on additive manufacturing technology are reviewed. The design and preparation of metal mirrors for additive metal fabrication and the post-treatment of substrates are discussed. Then, through analysis, the preparation process and key technologies of additively manufacturing metal mirrors are summarized. Finally, prospects for additively manufacturing mirror applications are presented.