This paper first selects the theory of full lifecycle cost as the entry point and analyses the composition of full lifecycle cost of energy-saving buildings. Then it clarifies that the full lifecycle cost estimation of energy-saving buildings is an analytical tool in capital investment and a way to minimize the full lifecycle cost of energy-saving buildings. We also construct the concept and steps of cost estimation and build two mathematical models of full lifecycle cost estimation under two different application conditions; Finally, this paper illustrates the economic and social benefits brought by full life cycle cost estimation of energy-saving buildings through the case studies.