In this paper, the model of theoretical line loss calculation for high voltage network is studied. A new technique, also a new method of developing the graphic theoretical line loss calculation software for high voltage network is proposed, which uses Microsoft Visio2003 as development platform and Visual Basic for Applications (VBA) as secondary development tool. Based on the electrical connection diagram, the generally used method of graphic modeling of power system and the automatic identification technique of power network topology structure are studied. Finally, the practical graphic theoretical line loss calculation software for high voltage network is developed, which is operated under Visio2003. The development of this software indicates that the technique of secondary development for Visio offers a new effective method for the development of the graphic theoretical line loss calculation software for high voltage network and other power application software.