Variation of temperature is a primary environmental factor that affects the behavior of structures. Therefore, understanding the mechanisms of normal temperature-induced variations of structural behavior wouldhelp in distinguishing them from anomalies. In this study, we used the structural health monitoring data of theShanghai Yangtze River Bridge, a steel girder cable-stayed bridge, to investigate the mechanisms of thermallyinduced vertical deflection ( DT ) at mid-span of such bridges. The DT results from a multisource combination ofthermal expansion effects of the cable temperature (TCab ), girder temperature (TGir ), girder differential temperature(TDif ), and tower temperature (TTow ). It could be approximated by multiple linear superpositions under operationalconditions. The sensitivities of DT of the Shanghai Yangtze River Bridge to the above temperatures were in thefollowing order: TCab > TGir > TTow > TDif . However, the direction of the effect of TCab was observed to beopposite to that of the other three temperatures, and the magnitudes of the effects of TCab and TGir were found tobe almost one order greater than those of TDif and TTow . The mechanisms of the thermally induced verticaldeflection variation at mid-span of a cable-stayed bridge as well as the analytical methodology adoptedin this study could be applicable for other long-span cable-stayed bridges.