A variable deck height continuous box-girder bridge is often built by its structural efficiency. When the aerodynamic stability of this kind of bridges is investigated by wind-tunnel test, a superposition method (strip theory) of results of different height cross sections is used to predict the response of the full bridge. However, past studies showed that the superposed result did not coincide with the full-model test result. In this study, a variable height box girder is simplified by a linearly changing height one and its vortex-induced vibration response is compared with the superposed response of different height cross sections. The result showed that large slope of the girder breaks the strip theory and an axial flow along the slope is attributed to the discrepancy.