Abstract: An accurate quantification and calculation of CO₂ emission in iron and steel manufacturing lays the foundation for analyzing and assessing its environmental impact and various emission reduction technologies. Based on the actual field data of Y steel mill in 2015, we calculated the CO₂ emissions per ton of Y steel mills by using domestically recommended input-output method and the life cycle method recommended by the World Steel Association. The result indicates that the CO₂ emission per ton of steel produced calculated by the life cycle method is 2.183 t CO₂, which is significantly higher than the CO₂ emission of 1.940 t CO₂ calculated by domestic input and output method. This is mainly due to the fact that the calculation method proposed by the World Steel Association is more inclusive in statistics and takes into consideration the carbon dioxide emissions generated in upstream materials and energy. Based on the input-output method and the life cycle method, we propose a kind of greenhouse gas emission calculation method based on the LCA method that meets the national realities of Chinese steel companies from the perspectives of calculation boundary, emission factors, material and energy classification and evaluation baseline.