Abstract: To accurately assess the carbon emissions associated with assembled steel structure office buildings, this study develops a comprehensive life cycle carbon emission calculation. Utilizing advanced tools such as Autodesk Revit, Tekla Structures, and DeST-C, the model offers a detailed methodology for calculating emissions. This approach adheres to the guidelines set forth in the “Standard for the Calculation of Carbon Emission from Buildings” (GB/T 51366—2019). By applying this model to the Wuxi CEC PARK case study, the research quantifies the carbon footprint of such structures and compares these findings with those of assembled precast concrete structures and cast-in-place steel–concrete structures. The model encompasses all six stages of a building’s life cycle: material preparation and component production, logistics, operation and maintenance, renovation and reuse, and finally, dismantling and recycling. This comprehensive scope ensures that the model captures the carbon emissions linked to every aspect of “human, machine and material” interactions throughout the building’s life cycle. The accuracy of the calculation results is thus guaranteed. Specifically, the analysis highlights the carbon emission factors from steel structure components within the context of Wuxi CEC PARK. The results show that the average annual carbon emission intensity for the building is 59.47 kg·m⁻²·a⁻¹. This figure is approximately 5.46 kg·m⁻²·a⁻¹ and 13.59 kg·m⁻²·a⁻¹ lower than that of the assembled PC structure and the cast-in-place steel–concrete structure, respectively. These results underscore the beneficial impact of assembled steel structures in reducing carbon emissions. The operation stage is the largest contributor to carbon emissions, accounting for 58.79% of the total. This is followed by carbon emissions from material preparation and component production, which represent 29.58%. The study finds a negative carbon emission value of −3295.27 t in the dismantling and reuse phase, indicating a significant reduction in emissions in this phase of the building’s life cycle. Additionally, the incorporation of renewable energy sources and plant carbon sinks into the building’s design further offsets carbon emissions, contributing an additional reduction of 389.33 t. The method proposed in this paper addresses the gap in existing methodologies for full life cycle carbon emission calculations for assembled steel structure office buildings. It provides a solid foundation for such buildings, promoting the adoption of assembled steel structure office buildings. It lays a foundational framework for assessing the carbon reduction potential of these structures, thereby facilitating their wider adoption within the construction industry. It provides a reference for future research on the prediction of carbon emissions for similar office buildings and advancing carbon reduction initiatives in the construction industry.