Abstract. Building energy conservation is a worldwide tendency. Due to the limitation of energy resource, although all countries have done a lot of researches on discovering new energy resource continuously, which is not enough to mitigate the tension of energy supply in a short time. As a result, many countries have paid attention on saving conventional energy, and "energy-saving" is called the fifth energy after coal, oil, natural gas and nuclear power. This paper mainly summarizes the background and meaning of building energy conservation, especially expatiates the theoretical method of thermal conductivity of composite wall in energy saving, including the research contents of domestic and foreign researches and the specific theory of thermal conductivity of composite wall. It aims to theoretically provide guidance on the energy-saving of composite wall.