1Cr18Ni9 steel springs are widely used in aerospace equipment due to their excellent performance. However, the significant influence of temperature on their stress relaxation properties has become a key factor leading to spring failure, thereby threatening the stable operation of the mechanism. Therefore, in-depth research into the stress relaxation characteristics of these springs is crucial for ensuring their stable and reliable operation. In this study, 1Cr18Ni9 steel springs were selected for stress relaxation tests to investigate relaxation characteristics. Based on the experimental data, an Arrhenius model was applied to establish a spring service life prediction model. The model calculations indicated that the load loss rate after 15 years of service was 8.9%, meeting the design requirement of < 10%; combining transmission electron microscopy (TEM) techniques, the microstructural evolution of the spring after stress relaxation at different temperatures was observed. The study found that as temperature increases, the stress relaxation rate of the spring accelerates. The number of twins in the microstructure first increases and then decreases, while the number of dislocations first decreases and then increases. Throughout the stress relaxation process, no phase transformation occurs in the crystals. This study reveals the correlation between macro- and micro-stress relaxation behavior of 1Cr18Ni9 steel springs under temperature-driven conditions, providing important application guidance and theoretical support for the safe and reliable operation of such springs in complex temperature environments.