Off-axis reflective optical systems have been widely used due to the nature of miniaturization and no obscuration. However, it is difficult to control the desired system structure during the designation and optimization, especially to maintain the compact system structure. In this paper, a constructing method of off-axis reflective optical systems is proposed to achieve the compact feature. The structure construction combines the constraints of the aberration and the system structure. Multiple parameters of aberration (optical performance) and the mirror coordinates (system structure) are simultaneously optimized to obtain the initial structure using the genetic algorithm. The initial structure is compact, unobscured, and remains less difficult to optimize. A compact off-axis three-mirror reflective system was designed with an entrance pupil diameter of 90 mm, a focal length of 405 mm, and a field of view of 3∘×3∘. Each mirror is expressed by an XY polynomial to achieve optimized optical performance.