<p>To systematically quantify the life cycle carbon emissions of section-enlarging reinforcement method for reinforced concrete structures in existing buildings, this study employs a retrofit project of a national-level research office building to study these emissions. Firstly, carbon accounting boundaries for production, transportation and construction stages of building materials is defined. Then, a life cycle assessment (LCA) calculation model is constructed based on the emission factor method. The carbon emissions of each stage are analyzed and energy consumption parameters during transportation and construction. Finally, a series of emission reduction strategies are proposed. Results show that the emissions from building materials production are the main source of carbon emissions, accounting for 93.6%. The carbon emissions in the transportation and construction stages account for 5% and 1.4% respectively. This study provides data support and strategic references for low-carbon retrofit of existing buildings. And it is also of great significance for realizing the “dual carbon” goals in the construction industry.</p>