On the basis of analyzing the demand data classification of grid theoretical line loss calculation and the inherent incidence relation of data, this paper proposes a grid theoretical line loss data synthesis method which takes mapping data and measure data as the data dependence mapping center. Combined with the source constitute of demand data of regional grid theoretical line loss calculation, a detailed process of data synthesis of regional power demand theoretical line loss calculation method is deduced. The synthetic method supplies a theoretical technique support for the full realization of automatic data acquisition, automatic synthesis, automatic calculations and the implementation of online calculation and analysis for the theoretical line loss. At present, the synthesis method has been implemented and successfully run in Foshan Power Supply Bureau of CSG.