The invention provide a theoretical line loss calculating method and system of AC-DC high voltage transmission network. The method comprises the steps that a data management module obtains EMS on-linedata and establishes an AC-DC power grid model; an analyzing and calculating module determines trend data files; a result gathering module calculates power grid theoretical line loss; a quantitativegeneration module forms a theoretical line loss table. The invention is advantageous in that automatic extraction and automatic calculation of data can be realized; the result table can be automatically generated and displayed; the power grid loss of a power transmission network can be assessed in a fine and effective way.