To clarify the influence of high-temperature steam curing of steel-ultra-high performance concrete (UHPC) lightweight composite bridge deck paving on the temperature field of the steel box girder, taking a certain bridge in Hunan Province as an example, finite element numerical simulation and actual bridge testing were conducted on the temperature field of the steel box girder during the high-temperature steam curing process of the bridge deck paving. The numerical calculations were generally consistent with the test results. The analysis results indicate that under the superimposed effect of solar radiation heating and high-temperature steam curing heating, the maximum temperature of the top plate of the steel box girder can reach 85.5 ℃, and the maximum value of temperature gradient is 51.5 ℃, far exceeding the maximum vertical positive temperature difference due to solar radiation specified in the General Specifications for Design of Highway Bridges and Culverts (JTG D60—2015). In steel box girders constructed with high-temperature steam curing of steel-UHPC lightweight composite bridge deck paving, the adverse effects of temperature gradients on the structure under the superimposed conditions of high-temperature steam curing and solar radiation temperature should be considered. In the early stage of high-temperature steam curing of steel-UHPC lightweight composite bridge deck paving, the maximum measured temperature and temperature gradient of the flat steel box girder depend on the high-temperature steam curing; therefore, the occurrence time of the maximum temperature and maximum temperature gradient depends on the duration of high-temperature steam curing, rather than the midday period with the maximum solar radiation. While in the later stage of high-temperature steam curing and after the completion of high-temperature steam curing, the maximum measured temperature and temperature gradient primarily depend on the solar radiation intensity. Different regions, seasons, bridge locations, and construction conditions may cause certain differences in the temperature of the steel box girder; the related issues of the temperature field of the steel box girder during the steam curing construction process of the steel-UHPC composite bridge deck are worthy of further research.