In order to study the influence of convective heat transfer boundary conditions on the temperature field distribution of steel box girder, a numerical simulation approach based on the modified convective heat transfer coefficient was proposed. Firstly, the commercial software Fluent was employed to simulate the wind velocity field of the steel box girder. Secondly, the convective heat transfer coefficient of the steel box girder computed and obtained with the Fluent was modified, and the temperature field distribution of steel box girder under different working conditions was further comparatively studied through finite element software ANSYS. The computation results of wind field on the surface of steel box girder show that compared to the inlet wind velocity, the wind velocities near the exterior surfaces of the steel box girder all decrease in different degree. Specifically, the wind velocities near the upper plate of the leeward side fairing and the bottom plate are reduced by 95.5% and 17.7%, respectively. The results of temperature field of steel box girder indicate that after introducing the modified convective heat transfer coefficient, the temperature differences in the steel box girder increase both vertically and horizontally. The vertically temperature difference will further increase after taking the auxiliary facilities on the bridge deck into the numerical simulation. Bridge deck pavement has little influence on the vertical temperature difference of steel box girder except the delay of the occurrence time of maximum temperature difference by 1 hour. In addition, the diaphragm can reduce the vertical and horizontal temperature differences of the steel box girder, due to its superior thermal conductivity. Therefore, the calculation of the temperature field of steel box girder should not ignore the influence of wind field near the surface of steel box girder, auxiliary facilities of bridge deck and diaphragm plate on the temperature field of steel box girder.