A long-wavelength infrared optical system suitable for fast turntables is designed to meet the needs of fast and small target detection and tracking. The optical system consists of a main optical system and a projection lens assembly, it adopts a reflex structure, a secondary imaging technology and a pupil matching technique, reduces the radial size and lateral dimension of the system effectively. The envelope of the shape is within the range of Φ204mm×360mm, compact structure and small volume. Considering that the system's working distance is in the range of 0.2km∼1km and the target speed is fast, the object distance has been specially designed to ensure that no focus adjustment is required within the range of action distance. In addition, stray light analysis is performed on the system and a stray light suppression scheme is given. The system is analyzed by stray light analysis software. The design results show that the optical system has excellent imaging quality and stray light suppression effect. Copyright © 2018 SPIE.