Construction management in China has embraced a new practice encompassing the evaluation criteria of green construction. Thus, carbon dioxide (CO2) emissions produced during the construction period should not be overlooked anymore. A quota based CO2 emission calculation method was established. A commercial building and a school building, as two cases were studied. It’s shown that the unit CO2 emission for the two buildings’ construction process are 11.5 kgCO2/m2, and 11.6 kgCO2/m2, respectively. The CO2 emissions for earthwork, foundation work, masonry, concrete work, reinforcement work, formwork, decoration work and measurement work during construction were calculated. The results showed that foundation work and measurement work emitted the most CO2, contributing more than 50% CO2 emission, following by reinforcement work and concrete work. Formwork and masonry work emitted very little CO2. The CO2 emission of different machines were also studied, and the results show that the top 10 machines can take up about 90% of the total CO2 emission. Tower crane, electric welding machine, drilling rig, pile driver, truck, excavator and concrete pump are some of the most CO2 emission machines. These results could help construction managers identify low carbon construction methods and machines in order to maximize the carbon reduction opportunity during the construction stage.