To reasonably quantify the carbonemissions level of constructing a metro shield tunnel and measure its emission reduction potential, the lifecycle assessment method is adopted to evaluate the carbon emissions during the construction of a metro shield tunnel and quantitatively analyze the intensity and level of carbon emissions based on actual engineering data. Thereafter, based on the scenario analysis method, the carbon reduction potential is explored from the perspective of promoting green building materials and clean new energy. The research results reveal that the carbon emission intensity of a singlering prefabricated segment is ~7.1 tCO2e/ring, whereas that of constructing a unit of metro shield tunnel is ~1.1×104 tCO2e/km. The carbon emission intensities of the material and prefabricated segmentprocessing phase, transportation phase, and shield tunneling and segment installation phase respectively account for 72.7%, 1.9%, and 25.4% of the total emissions of the entire construction. Although further utilization of recycled building materials and improvement of the clean energy structure can reduce the average annual carbon emissions by~6%, the cumulative carbon emissions from 2022 to 2035 will add up to ~1.3×107 t CO2e, which is equivalent to the national metro operation emissions in 2020.