Abstract: To satisfy the application requirements of large-field infrared monitoring, a passive athermal long-wave infrared (LWIR) fisheye lens optical system was designed based on aberration and fisheye lens design theory. The system has a focal length of 1.4 mm and achieves excellent imaging with a 200° fisheye field of view under a relative aperture of 1.14, matched with a 384×288 long-wave uncooled detector. According to simulation results obtained using ZEMAX (optical design software), the relative illuminance of the system is better than 97%, the f-θ distortion is less than 1.2%, and the Modulation Transfer Function (MTF) is greater than 0.4 at 33 lp/mm. A passive optical compensation athermalization design was adopted to achieve stable imaging quality over a temperature range of –40℃ to +60℃. In conclusion, the system features a large relative aperture, a wide field of view, high relative illuminance, low cost, and a wide temperature adaptation range. It can therefore be widely applied in large-angle monitoring fields such as astronomical meteorology, environmental monitoring, and public security and border defense.