To effectively monitor fast-moving aerial targets such as drones, timeliness is essential. Therefore, infrared optical systems require a large field of view and fast response times to quickly detect, track, and measure targets. This demand has driven the need for relatively large-field-of-view infrared surveillance cameras. Uncooled infrared detectors offer significant advantages in target detection: they do not require additional cooling equipment, which reduces costs and maintenance complexity, and they typically provide faster response times, enabling timely capture of target changes. To achieve relatively large-field infrared monitoring of aerial targets like drones, this paper presents the design of a large-field-of-view freeform off-axis three-mirror infrared imaging system based on uncooled long-wave infrared detectors. Compared to traditional systems, this design not only expands the field of view but also enhances monitoring efficiency, making it particularly significant for the surveillance of targets such as drones.Based on the design requirements (Tab.1) and using an uncooled long-wave infrared detector (Tab.2), this paper presents the design of a large field of view freeform off-axis three-mirror infrared optical imaging system (Fig.2). Prior to the design, a mathematical analysis of key optical parameters such as focal length, field of view, resolution, and entrance pupil diameter was conducted. The surfaces of the three freeform mirrors are represented using 7th order XY polynomials (Fig.3, Tab.3), and analyses were performed on the full-field geometric spot radius, wavefront aberration, modulation transfer function, and distortion grid (Fig.4-8). Finally, a Monte Carlo algorithm was employed for tolerance analysis to assess the impact of assembly errors of the three mirrors on imaging quality (Tab.4).The optimized freeform off-axis three-mirror optical system provides an extensive field of view of 7.8° × 5.6°, with a maximum geometric spot radius of 3.26 micrometers, which is notably smaller than the airy spot radius (Fig.4-5). The full-field wavefront error is measured at 0.031λ at 10 micrometers, approaching the diffraction limit (Fig.6), while the modulation transfer function (MTF) at 30 lp/mm is greater than 0.455 (Fig.7). The maximum distortion occurs in the lower right corner of the image plane, approximately 2.8%, remaining below the 3% threshold (Fig.8). Taking into account the assembly tolerances of the three mirrors (Tab.4), the expected geometric spot radius for the optical system is about 6.1 micrometers with a 90% confidence level. Overall, the imaging quality analysis confirms that the design meets the specified technical requirements (Tab.1).This research details the design of a large field-of-view off-axis infrared optical system, featuring an uncooled 800 pixel×600 pixel resolution long-wave infrared detector. With a focal length of 103 mm, the system achieves an impressive field of view of 7.5°×5.6° and an F-number of 1.47, demonstrating exceptional imaging performance. Our comprehensive analysis reveals that the imaging quality closely approaches the diffraction limit, as indicated by modulation transfer function (MTF) values exceeding 0.45 at a spatial cutoff frequency of 30 lp/mm across the entire field of view, while maximum distortion remains below 3%. Additionally, the size of the spot diagram is significantly smaller than a single pixel, further enhancing image quality. This advancement positions the design as a significant improvement over conventional infrared imaging systems. The combination of an expanded field of view and enhanced imaging fidelity makes this optical system particularly well-suited for applications in aerial target surveillance and infrared remote sensing, especially within unmanned aerial vehicle (UAV) contexts. Overall, this work establishes a solid foundation for future advancements in infrared imaging technology, with promising implications across various specialized fields.