Abstract: In design specifications for highway bridges and culverts of China, no solar thermal gradient is provided for the steel box girder without pavement, as a result, the solar thermal gradient has to be measured on site, which is a heavy work. This paper established a three-dimensional model for a section of the JiangDong Bridge steel box girder without pavement using ANSYS. The temperature field was calculated according to principles of heat conduction. The calculated temperature field at the scene of JiangDong Bridge coincides better with the measured one. Sensitivity analysis was carried out to study the effects of various parameters on the temperature field. The study shows that the absorptivity has the most significant effects on the temperature distribution, the wind speed and the emissivity also have some effects. Based on the measured data of JiangDong Bridge, the formula of exponential function and the linear function are adopted to model the vertical temperature gradient of the steel box girder without pavement. It proves that the fitting formula can also be applied to the solar thermal gradient of DongSha Bridge steel box girder, and can be applied further to the solar thermal gradient field of similar bridges in China. It is finally pointed out British Standard BS5400 may introduce large errors in analyzing the solar thermal gradient field of bridge structures in china.