On the basis of optical design theory, this paper designs a small-scale and dual-field optical system with a half sphere tracking field. This optical system takes a Code optical path as the main system and consists of three parts: tele-system at front, ray tuning system in the middle, and imaging system in the back. The first part is a telescope compound for adjusting the diameter of the light beam; the middle part is Code optical path made up four mirrors, which is used to turn the direction of the light beam; and the last part is an imaging system for long focal and short focal imaging and for tracking and recognizing targets. The focal lengths of the system are 60 mm and 120 mm and their Modulation Transfer Functions(MTFs) are all above 0.5 at 5.8 lp/mm. By imaging tests, this optical system has better imaging quality and can capture the images form the large field and small one at the same time.