According to the principles of various methods and energy consumption characteristics in Chongqing， new method of energy saving calculation is proposed for public buildings. It's shown that building envelope， facilities and energy management play the key role in energy saving reconstruction. Considering the difficulty of measuring adjustment quantity， the calculation should be under some certain prerequisites (design conditions)， also the prevailing industry standards can be fully utilized. During the energy saving calculation， another key point is how to reflect the contribution rate of the single technologies for energy saving reconstruction.