China National Petroleum Corporation (CNPC), China Petrochemical Corporation (Sinopec) and China National Offshore Oil Corporation (CNOOC) are not only big energy producers, but also big energy consumers, and play important roles in the national economy. Energy conservation target responsibility assessment is an important way to promote energy conservation and emission reduction of enterprises. In this paper, an energy conservation evaluation index system of large oil company is established, which combines the quantitative index and qualitative index. With this index system, 10 subordinate enterprises of an large oil company are evaluated and graded. It provides support for oil companies to meet national energy conservation assessment requirements and improve energy conservation management level.