Abstract: To investigate the secondary stress at the details of orthotropic steel decks (OSD) induced by thermal gradient in steel box girders, the temperature field of the steel box girder of a self-anchored suspension bridge is measured for multiple times under high environmental temperature and strong solar radiation. The vertical temperature gradient is fitted based on the measured maximum temperature difference between the roof and the floor. After establishing the sectional box girder model in ANSYS with the measured temperature applied on the box-girder surface, the temperature field in the sectional model is obtained. The temperature results on the floor beam agree well with the measured temperature, which validate the thermal analysis. Based on the simulated 24 h temperature field, the thermal stress field in the sectional box girder is first analyzed. Refined stress results are obtained based on a sub-model technology. The thermal stress time histories are determined at the four details around rib-to-floor beam (RF) connection and the cutout detail. It is found that, under strong solar radiation and high environmental temperature, the transverse temperature difference in the steel-deck box girder is not apparent, while the vertical thermal gradient is significant and can be fitted as a four-broken-line function with the maximum temperature difference lower than that of the Eurocode. Significant stress concentration appears at the details of the OSD, particularly at the cutout detail. The cutout detail will be fatigue-free if the thermal stress range resulting from the vertical temperature under solar radiation is considered, or if the stress range resulting from the truck loading is considered. The stress range at the cutout detail, which is jointly produced by the thermal effect of the vertical temperature and by the truck loading, is larger than the constant-amplitude fatigue limit and may contribute to the fatigue crack at the cutout detail.