In recent years, ultra-high performance concrete (UHPC) has been widely used. However, due to excessive cement consumption in the production process of UHPC, the solid durability and long life of UHPC, the level of carbon emission from UHPC is not yet well understood. Therefore, the life cycle assessment (LCA) method was used to build a quantitative UHPC emission analysis model to compare and analyze the carbon emissions in the whole life cycle between steel-UHPC bridge deck and conventional steel-concrete bridge deck structure. The results show that although the carbon emission of UHPC in the production stage is about 1 245.84 kg CO2eq/m3, which is 1.58 times that of the ordinary concrete, in terms of performance, the carbon strength of UHPC is 62.25% of the ordinary concrete, so UHPC is a greener building material. Over the entire life cycle, the average annual carbon emissions of steel-UHPC bridge decks decrease by 35.76% than conventional steel-mixed bridge deck, offering significant carbon reduction potential and contributing to the sustainability of infrastructure development. The steel-UHPC bridge deck scheme has engineering comparison advantages and excellent economy, and its carbon emission per unit output value is 0.89 t CO2eq/103 yuan, which is 86.41% of that of the conventional steel-mixed bridge deck and has a better carbon reduction effect. The reliability of the results and further emission reduction measures are discussed, and finally, the contents to be further studied are put forward.