Abstract: A novel mathematical model for the large scale extra high voltage AC/DC systems is presented in this paper. This general mathematical model consists of AC system and DC system, which can avoid the uncertainty of traditional alternate calculation and power equivalence method and directly reflect the coupling relationship between the feature of DC network and AC/DC variables. The interior point method for solving this model is also proposed. Extensive numerical simulations on systems with 118 to 7060 nodes have shown that this method is a good analysis and optimizing tool of electric power network due to its fast calculating speed and easy convergence, and it is also a fundamental application module that can provide data for the advanced software of power system.