To achieve the comprehensive assessment goal of economy and energy conservation in energy-saving design scheme selection, based on the life cycle theory, exergy analysis method and thermo-economics, the exergy consumption and CO2 emissions of different energy-saving design schemes and exergy cost models were evaluated and built. The analysis of the two energy conservation schemes of an office building in Zhuzhou indicates that, to reach the true sense of energy conservation, energy saving and CO2 emission cuts in building operation stage should be greater than the consumption in building production stage. The energy consumption and CO2 emission in the building production stage of Scheme one and Scheme two will need 10, 11 years and 4，5 years respectively to be offset. And the exergy consumption in the production stage is far more than the exergy saving in the operation stage. The analysis result of exergy cost reveals lower exergy cost means less investment, more low-grade energy usage and higher exergy efficiency. It shows the energy saving property and sustainability of buildings.