Energy efficiency in buildings is a worldwide problem, especially in China, it is under developed and begins very late, so research on energy efficiency theory in buildings is a pressed problem. In this paper, a new understanding for BEE is put forward and the character of the social activity is emphasized with building energy consumption process. The new definition embodies the systematic characters of subject, object, and sense of principle and target of BEE technology and practice, which is useful to establish the foundation of the systems theory of knowledge. This paper also inquires into the problems and straits around to BEE. In order to improve the energy efficiency theory, to produce more suitable environment of human settlements as well, we should establish the systems theory, human and culture view, technology theory and regional view. To accelerate the process of research and development of energy efficiency in buildings in China, this paper conducted a theoretical research with the building environment, energy utilization and human development. The key to establish firmly building theory of energy efficiency is how to exploit the local natural conditions and stick to the requirements for the harmony of human being with environment; this paper sets up the preliminary system frame in the end.