To meet the increasing demand for practicality and rapidity of the optical components in aircraft folding optical systems, an aluminum mirror preparation method based on additive manufacturing (AM) technology is proposed. From the perspective of special processing technology for AM, a preparation process chain of additively manufactured mirrors is proposed to address limitations in the design process such as size, support, and connectivity constraints. First, the deposited mirror manufactured by AM technology is prepared. Second, the additively manufactured metal mirror is densified by the hot isostatic pressing process. Then, computer numerical control machining and single-point diamond turning is applied to realize the shape of the surface. Finally, the AM mirror is coated with the gold film. The experimental results show that the surface quality is 0.384λ (peak-to-valley method) and 0.093λ (root mean square method) (λ  =  632.8  nm), and the mirror surface roughness (Ra) is better than 8 nm. After the mirror is subjected to thermal cycles, the surface accuracy does not change significantly, which essentially meets the requirements of the aviation environment adaptability and shows good potential for application in the infrared band.