Large-span spatial steel structures are subject to complex stress conditions during construction, especially during the unloading phase, and they are easily affected by the environment and the loading method. Therefore, it is necessary to analyze the influence of different unloading construction methods on the structural reliability during construction to improve the quality of construction safety. In this paper, ANSYS version 16.0 software is used to establish a finite-element analysis model of the truss structure of the Harbin railway station. The synchronous unloading process and asynchronous unloading process are numerically simulated, and the stress variation trend of the arch truss during the unloading stage is studied. The research results show that the effect of simultaneous unloading on the mechanical state of the structure is small. During the unloading process, stress is more likely to exceed its limit in the area where the boundary conditions change. Then axial strain monitoring was performed during the actual unloading process of the truss structure of the Harbin railway station. The results show that the effect of synchronous unloading on the structure's stress state is small, which is in good agreement with the numerical simulation results.