Carbon accounting is critical to manufacturing and achieving a low-carbon transition and lean carbon management. A comprehensive understanding of carbon emissions in manufacturing is essential to calculate a product’s carbon footprint accurately. Based on the life cycle assessment (LCA) method, this study divides the whole process of mechanical products from parts to finished products into three stages: parts (P), assembly (A), and testing (T). By decomposing each stage’s carbon emission sources and combining each stage’s characteristics, a series of corresponding carbon emission accounting models is established. Finally, the three-stage carbon emission model of the manufacturing process of a three-piece ball valve is established, and the validity and feasibility of the proposed model are verified. The results show that raw material consumption, energy consumption, and transportation are the primary sources of carbon emissions in the manufacturing process of three-piece ball valves, accounting for 35.6%, 38.8%, and 17.6%, respectively. The corresponding carbon emissions were 17.854 kgCO₂e, 19.405 kgCO₂e, and 8.8 kgCO₂e, respectively. Through these results, we can provide some theoretical and data support for the low-carbon transformation of manufacturing enterprises as well as some research ideas for realizing low-carbon production through process planning and shop scheduling.