The article divides the electromechanical transient simulation of large AC and DC power grids into AC network and DC network. With reference to the modeling method of AC network, all DC transmission systems are processed as a DC network for unified modeling and simulation. The modeling of the DC transmission system is extended from the components embedded in the AC system to the dimension of the DC network. The AC and DC networks exchange information at the commutation buses. Through the establishment of mathematical models of LCC commutation nodes, VSC commutation nodes, DC loads, DC lines, etc., the formation and solution methods of the DC network equations are explained in detail. The proposed method is simulated through the example system. The results show that, the proposed method can effectively adapt to many different types of HVDC power transmission forms.