Abstract: The mid-span vertical displacement is an important index in structural health monitoring, and it changes considerably with the temperature variation. Based on field measurements of the Shanghai Yangtze River Bridge (SYRB), this study investigated the mechanisms of temperature effects on the mid-span vertical displacement of two-tower cable-stayed bridges through the plane geometric analysis and the finite element analysis, with the purpose of understanding the behavior of such bridges. It is found that the thermally induced mid-span vertical displacement (D_T) does not vary monotonously with the temperature of atmosphere or a particular structural component. The D_T could be well approximated by a multiple linear superposition of thermal expansion effects of the cable temperature (CT), girder average temperature (GAT), differential temperature between top and bottom plates of the girder (GDT), and tower average temperature (TT). For SYRB, the effects of CT and GAT on the variation of D_T are almost one order greater than those of the GDT and TT, and the direction that D_T takes in response to CT is opposite to that of the other three temperatures. The simplified plane geometric model proposed in this paper is able to estimate the D_T of cable-stayed bridges preliminarily.