Structural thermal stability is one of the important indexes of spacecraft structural design, which plays an important role in realizing the function and performance of spacecraft in space environment. In this paper, the thermal stability test method of large spacecraft structure under vacuum and low temperature is studied. Based on the principle of optical photography deformation measurement, the construction of the optimal deformation measurement field for large structures is realized through the innovative design of the measurement system architecture. The automation and reliability level of deformation measurement are improved through the control design of the measurement system. The system validation test and accuracy analysis method are researched. The test results show that the system can realize the transient and high-precision deformation measurement of the structure size more than 4 meters under the vacuum and low-temperature environment. The system is stable and reliable, and provides an important technical means for the correctness verification of spacecraft structure design.