为了设计出适用于太赫兹波段成像的光学系统，综合分析了各种成像系统的特点，其中离轴三反射式光学系统具有独特的优势。通过对离轴三反射式光学系统的设计原理、设计方法和步骤的详细介绍，利用ZEMAX光学设计软件，通过编写简单的ZPL宏指令以及光阑和视场离轴，设计了工作在太赫兹波段、入瞳大小250 mm、瞬时视场1 mrad、焦距800 mm、F数3.2的系统，实现了可用于太赫兹波段无中心遮拦的离轴三反射式准光学成像系统设计。

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.