It is well known that green retrofitting of existing buildings is key supporting research of China's “Twelfth Five-Year Plan”. To undertake and promote retrofitting of existing buildings, especially green retrofitting in China, the primary problem needed to be solved is diagnosis for existing buildings. Just through diagnosis, the present situation of existing buildings can be completely mastered so that the subsequent green retrofitting can become more targeted. In this paper, study for the index system of diagnosis of green retrofitting of existing buildings is discussed. This paper also illustrates the principle of the establishment of the index system of diagnosis. In the meantime, the whole index system of diagnosis, the whole diagnosis framework, procedure and method are also given. All these give a guidance for the diagnosis of green transformation of existing buildings which is significant for technology research of green retrofitting of existing buildings in China.