In recent years, ballastless track systems in high-speed railways have developed rapidly, but studies on carbon emission characteristics and precise calculation methods during the construction phase remain limited. This paper conducts a comprehensive quantitative analysis of carbon emissions during the construction phase of CRTS Ⅲ slab ballastless track in high-speed railways. The PROMETHEE-Ⅲ method is used to rank carbon emission factors, and a ballastless track construction carbon emission database is built using MySQL. The proposed models quantify the per-kilometer carbon emissions of embankment, bridge, and tunnel sections. Concrete accounts for 41.06%~43.52%, steel accounts for 52.90%~54.53%, with materials contributing up to 96.00% of total emissions. Results show that embankment sections produce significantly higher carbon emissions than bridge and tunnel sections. Concrete and steel are the dominant emission sources. Sensitivity analysis reveals that steel emission factors have the strongest impact on total emissions, followed by concrete.