Aiming at the problems of temperature sensitivity, setting at room temperature, defocusing at low temperature, and low accuracy of camera focal surface prefabrication at room temperature, a defocusing test technology of low temperature infrared optical system based on opto-machine co-simulation, interferometry, and linear displacement measurement was proposed. The main factors causing low temperature defocusing are analyzed, and the defocusing data is calculated by optical machine simulation. The low temperature interference test optical path of the optical system is established by using the sensitive characteristic of power to focus position, and the low temperature defocusing of the infrared optical system is tested by combining high precision displacement measurement. This technology is used to analyze and test the defocus of a light and small infrared camera with an 181 mm aperture from normal temperature to low temperature. The deviation between the test results and the simulation calculation is less than half of the system focal depth. On this basis, the camera is preset at a normal temperature focal plane, and the experiment shows that the preset focal plane is accurate, which proves the feasibility and accuracy of the low temperature defocus measurement method. The test method can be used for presetting and focusing light and a small optical camera at normal temperature which is sensitive to temperature.