In order to study the environmental benefits of urban rail transit projects brought by carbon emission reduction, construction and operation data were integrated to construct a carbon payback period calculation model for urban rail transit projects. Taking 5 rail transit lines in Changsha city as the research object, based on a reasonable forecast of future passenger traffic, the carbon footprint changes of these 5 lines during construction and operation from 2014 to 2050 were quantitatively analyzed. It is found that the urban rail transit project has significant carbon emission reduction benefits. All five lines can fully recover the carbon emissions of the construction stage by 2050, and afterwards, carbon emissions trading will also generate positive benefits. At the same time, it maintains that the carbon emission reduction benefits of rail transit projects are directly affected by the saturation of passenger traffic.