Externality exists in the field of existing commercial residential buildings energy-saving retrofit which hinders the implementation of energy-saving in-depth. To build the externality calculation method in order to provide the necessary basis for the internalization of externality and the establishment of energy-saving externality repayment mechanism. To describe the causes and manifestations of energy-saving externality by economic theory and method. To propose the calculation method on the basis of identifying the externality donors and acceptors. To demonstrate the applicability of the calculation method externality through case analysis. The conclusions show that the externality should be reduced or eliminated; the necessary means should be adopted to internalize the externality of energy-saving retrofit by building a unified external energy saving calculation method, establishing energy-saving retrofit externality repayment and energy savings trading mechanism.