For long-span suspension bridges, the structural deformation is significantly influenced by the environment. Therefore, evaluating the structural stiffness of suspension bridges is crucial. This paper takes a certain long-span suspension bridge over the sea as an example to discuss different methods for evaluating the structural stiffness of suspension bridges. A three-dimensional finite element model is established, and influential lines at key locations are extracted. The fitting accuracy of vertical deformation and environmental temperature data under different time windows is analyzed based on global positioning system (GPS) deformation monitoring data. Furthermore, the spatial distribution of the maximum and minimum values of bridge deformation caused by vehicle loads is statistically analyzed. The results indicate that the displacement extreme value coefficient at the mid-span of the suspension bridge is relatively large compared to those of other locations. The measured displacement envelope curves are within the theoretical envelope curves, demonstrating good overall stiffness performance of the bridge. The research methodology in this paper accurately evaluates the overall structural stiffness of the suspension bridge, providing strong support for ensuring the safe operation of the bridge.