As an important tool in medical diagnosis and treatment, endoscope has been widely used in modern medical technology. Especially in minimally invasive surgery, endoscopy can provide doctors with real-time image information, improve the accuracy of the doctor's operation and treatment effect, and effectively reduce the risk of surgery. Among them, visible light endoscopy system as the core diagnostic tool, still maintains a dominant position. However, for lesion screening, although visible light can provide doctors with clearer images, there are some limitations: first, the imaging ability of visible light endoscope in low brightness environment is limited, which can not meet clinical needs; Secondly, for the early diagnosis of malignant masses or tissue inflammatory lesions, visible light band can not effectively reveal the information of potential lesions. Studies have shown that when malignant masses or tissue inflammatory lesions occur in the body, local diseased tissues or organs increase local blood flow due to accelerated sugar metabolism and vascular dilation, resulting in relevant changes in the thermal radiation of local tissues and organs, which will be manifested as an increase in body temperature. In addition, infrared thermal imaging technology has also shown important value in the field of medical diagnosis. The existing research group has made important progress in the treatment of early lung cancer. They successfully realized the visual identification of intersegmental plane in segmental resection by infrared endoscopy technology, providing a new technical means for accurate surgical treatment. In view of this phenomenon, infrared endoscopy technology can make up for the shortcomings of visible light endoscopy and provide more accurate support for early diagnosis of diseases. In this paper, an infrared endoscope system with 30° viewing angle is designed for minimally invasive pulmonary surgery. The clinical application scenario of this system is mainly for the operation of the peripulmonary area in the thoracic cavity. The system is composed of objective mirror module, relay mirror module and coupling mirror module. The operating wavelength is 8 ~ 14 μm, the F number is 1.39, the working distance is 100 mm, the total length of the system is 289 mm, and the MTF value of the full field of view is better than 0.2 at the spatial frequency of 41.7 lp/mm. In terms of material selection, only Ge and ZnSe infrared optical materials are used in the system, and two sets of optical elements with completely symmetrical structures are used in the relay mirror module. Through the optimization design, the planar optical surface is introduced into the objective lens module and the coupling mirror module, which effectively simplifies the optical system structure and significantly reduces the production cost. Considering that the body temperature and infrared radiation generated by the infrared system will make the image plane position offset, non-heat treatment is performed by active heat elimination. In addition, the feasibility of the system under machining and assembly errors is verified by tolerance analysis. This study provides theoretical support and technical guarantee for the further application of infrared endoscope in medical field.