The vertical temperature gradient (VTG) is a critical factor contributing to thermal stresses or deformations in bridge structures. However, the current provisions concerning the VTG of steel box girders are inadequate. Accordingly, this paper systematically investigated the VTG of a steel box girder in Wuhan using field measurement and numerical simulations. The temperature distributions of the steel box girder were first characterised based on measured temperature data. A thermal model of the steel box girder was then established using measured time-varying meteorological data, with the convective heat transfer coefficient obtained through an inversion method to update the model. The numerical findings align well with the observed temperature, and the fitted unfavourable VTG values exceed the current normative threshold. Furthermore, parametric investigations were carried out to reveal the impacts of meteorological and structural parameters on the VTG. The results indicate that solar radiation and pavement thickness have a considerable impact on the VTG. Finally, this research recommends that the present VTG guidelines should be modified by introducing appropriate adjustment factors related to the level of solar radiation intensity in different regions to strengthen their capacity for safeguarding.