Infrared detection technology has the advantage of passive thermal radiation detection and continuous work day and night. It can greatly reduce the restriction of environmental factors such as the light conditions. It is widely used in ecological environment monitoring, night vision detection, precision guidance and other fields. In recent years, with the development of infrared detection technology, especially in the field of aviation and aerospace remote sensing, in order to improve the timeliness of infrared remote sensing detection and realize the large-scale deployment of infrared detection system, the demand for large-field-of-view, high-compact, lightweight and low-cost infrared detection system is becoming more and more urgent. For this purpose, a freeform off-axis four-mirror all-aluminum infrared optical detection system with a large field of view and a compact package is designed in this paper.A freeform off-axis four-mirror all-aluminum infrared optical detection system is designed and built in this paper. The optical system has a real exit pupil to connect the cold aperture of a cooled infrared detector (Fig.2). The 7^th order XY polynomials is used to represent the surface of the four freeform mirrors. The full-field geometric spot radius, wave aberration, modulation transfer function, and distortion grid are analyzed (Fig.5-9). The Monte Carlo algorithm is used for tolerance analysis to determine the influence of the alignment errors of the four mirrors (Tab.3). The optical system adopts an all-aluminum optomechanical design (Fig.12), in which the aluminum freeform mirror employs a three-ear flexible support mode to reduce the rigid connection stress (Fig.13). The optical, mechanical and thermal integration analysis is carried out, and the athermal effect of the optical system is verified (Fig.14-16). The optical system is assembled, and the full-field wave aberration is measured (Fig.17-18). The optimized freeform off-axis four-mirror optical system has a large field of view of 6.25°×5°. The maximal geometric spot radius over the whole field of view is 5.36 μm, which is far less than the radius of the airy spot (Fig.5-6). The full-field wavefront error is less than 0.037λ@8.85 μm, which approaches the diffraction limit (Fig.7). The minimal MTF at 20 lp/mm is 0.48 (Fig.8). Considering the conventional alignment errors of the four mirrors (Tab.3), the geometric spot radius of the optical system is expect to be less than 19.8 μm. According to the optical, mechanical and thermal integration analysis, the maximal full-field geometric spot radius is slightly changed from 5.36 μm to 5.49 μm when the working temperature is changed from 20 ℃ to 30 ℃ (Fig.16). The result proves that the all-aluminum optomechanical system potentially has the optically athermal characteristics. The prototype has a focal length of 146.2 mm and a NETD of 26.8 mK. The measured wavefront error of the prototype is less than RMS 0.7λ@632.8 nm, which meets the technical requirements (Fig.18). A freeform off-axis four-mirror all-aluminum infrared optical system with a real exit pupil, a large field of view and a compact package is built in this paper. The optical system has a field of view of 6.25°×5°. The designed full-field geometric spot radius, wavefront error and modulation transfer function all approach the diffraction limit. The tolerance analysis of the alignment errors of the four mirrors is carried out based on the Monte Carlo algorithm, which leads to a full-field geometric spot radius of less than 19.8 μm. The optical system adopts an all-aluminum optomechanical design, which naturally possesses an optically athermal potentiality. The optical, mechanical and thermal integration analysis for a temperature rise of 10 ℃ of the optical system verifies the optical athermality of the optical system. The measured full-field wavefront error of the prototype is less than RMS 0.7λ@632.8 nm. The captured far field infrared image shows the high performance of the prototype. Compared with the traditional off-axis reflective optical system, the demonstrated optical system adopts a new configuration of "all-freeform optical surfaces + all-aluminum optomechanics". It can achieve a larger field of view with a more compact envelope. And, the system has the characteristics of lightweight, low cost and optical athermality, which has important application prospects in the field of infrared detection.