One of the main substance that contributes to climate warming is CO₂, and the construction industry accounts for about 25% of the world's total CO₂ emissions. During the life cycle of a building, the stage with the largest average annual CO₂ emissions is the embodied stage. Therefore, it is important to study CO₂ emissions from the embodied stage of buildings. At the same time, the uncertainty analysis method was used because of the partial variability of data such as emission inventories due to human factors and the limitations of the technology level. In this paper, we decompose the building by sub-projects, quantify the CO₂ emissions of the embodied stage by combining specific unit processes, and then give the unit processes that need the most attention in emission reduction. Through the case study, it is found that the focus of carbon emission reduction in the embodied stage of the building is the material production phase, which accounts for 93% of CO₂ emissions. Next, among the different processes, the focus of emission reduction is civil engineering, whose carbon emissions account for 85% of the overall. Two types of uncertainties, data and scenario, are also calculated in the carbon emission quantification process. According to the results of data uncertainty, it can be seen that the coefficient of variation of the total CO₂ emissions in the embodied stage is 0.3%, and the confidence of the result data is high. According to the results of the scenario uncertainty, it is known that changing the distance and type of diesel fuel in the transportation stage can reduce carbon emissions by 39%, and changing the type of electricity used in the construction stage can reduce emissions by 27%.