Infrared detection technology plays a crucial role in meteorological observation, ground detection, and astronomical observation. To enhance the sensitivity and reduce background noise of the infrared detector, it is necessary to lower the operating temperature. The radiant cooler has advantages of no vibration and no power consumption. It has been applied in various satellites to cool infrared detectors and optical components. To meet the cooling requirements, the radiant cooler has a complex mechanical structure. It is assembled at room temperature but operates at a low temperature condition. The thermal deformation of this complex structure causes relative displacement of the optical components, a phenomenon known as low temperature displacement, thus degrading imaging quality. The low temperature displacement of the radiant cooler should be obtained by calculation or measurement to serve as the basis for structure assembly. Aiming at the difficulty of measuring displacement within the complex structure using laser displacement sensors, an indirect measurement scheme is proposed.A measurement test system using equipment and laser displacement sensors is established in this paper. The laser displacement sensor measures the surface of the equipment extending from the complex structure of the radiant cooler, converting the measurement of displacement of surfaces inside the radiant cooler into the measurement of displacement of the equipment (Fig.2). The measurement system uses multiple symmetrically arranged laser displacement sensors to measure simultaneously displacements of 3 directions (Fig.2). Multilayer insulation and heaters were used to keep the temperature of laser displacement sensors between (20±2) °C, and other parts of the measuring system in the vacuum chamber (Fig.3). A low temperature displacement measurement test of the mid-wave channel of a W-type radiant cooler was carried out, and the translational components of the low temperature displacement are obtained.The sensor measurement value changes and low temperature displacement results in the X and Y directions were obtained (Tab.2). Sensor measurement values were obtained multiple times for the position of equipment in both normal condition and low temperature condition. Then the average values and standard errors were obtained for further derivation of low temperature displacement. The controlled temperature of sensors leads to a standard error of around ±10 μm. As a result, in the X direction, the low temperature displacement was 51 μm with a propagated error of ±11 μm; In the Y direction, it was -47 μm with a propagated error of ±24 μm. These results align well with the calculated values. The random error in sensors mainly arises from the sensor's linearity and the influence of temperature fluctuations. There are three types of system errors in the measuring system, which are caused by local thermal deformation of the sensors, the thermal deformation of the equipment, and rotation of the equipment (Fig.4). By averaging the values from opposing sensors on both sides, system errors can be reduced, thus enhancing the accuracy of the measurement results.A measurement method using the equipment and laser displacement sensors was proposed, providing a solution for measuring deformation in complex structures. An equipment extending from the complex structure was designed, low temperature displacement was measured by using opposing sensors and taking the average, and the temperature of sensors were controlled to be more accurate. The low temperature displacement measurement test was carried out on a W-type radiant cooler, with measured low temperature displacements of 53 μm in the X direction and 47 μm in the Y direction, and the measurement uncertainty were around ±11 μm and ±24 μm, respectively. This method significantly reduced system errors caused by sensor thermal deformation and equipment rotation, thereby improving the accuracy. Compared with eddy current displacement sensors, the use of laser displacement sensors eliminated the need for low temperature calibration, further enhancing measurement precision, and the relative error of this method was comparable to that of mentioned cases using photogrammetry in the vacuum environment.