With the continuous development of global industrialisation, the construction industry plays a key role in achieving the goal of 'double carbon'. Since there are fewer studies on steel-hybrid composite buildings, this paper takes a large public building in Jiaxing, Zhejiang Province, as a research object, and combines LCA and BIM technologies to measure the bill of materials and bill of quantities of a sample project. The carbon emissions of the building at each stage of the whole life cycle were calculated according to the formula, and the differences at each stage were compared. The results of the study show that the full life cycle carbon emissions of a large public building is 62, 509, 826.81 kgCO₂, of which the production stage and operation stage have the highest percentage of carbon emissions, which are 56.02% and 42.87%, respectively. During the production phase, steel is the main source of carbon emissions, which is 27, 217, 525 kgCO₂. This study provides a practical reference for calculating the carbon emissions of similar large-scale public composite buildings.