Abstract: In order to design an optical imaging system working in the terahertz range, a comprehensive analysis and comparison of various optical imaging systems are given, the character and unique advantages of off-axis three-mirror system are presented. With the principle, design methods and procedures of off-axis three-mirror system are analyzed, and simple ZEMAX programming language (ZPL) macros are created. Both the pupil and field of view are off-axis properly after optimization with ZEMAX software. Finally, an off-axis three-mirror system working in the terahertz range is designed with a pupil of 250 mm, the IFOV (instantaneous field of view) of 1 mrad, a focal length of 800 mm, and an F number of 3.2. Thus, the off-axis three-mirror quasi-optical system working in the terahertz range is realized.