The method of companion model is applied to the digital simulation of high voltage hybrid AC/DC systems for the first trme in this thesis.The mathematical model of the digital simulation of hybrid AC/DC systems is represented. This method can model the normal states, faults and games of faults of any hybrid AC/DC systems. The method has a series of advantages : saving the computation and the amounts of memory, high precision and fast calculation and it can be used with convenience. The simulation results of a hybrid AC/DC system arc in consistent with that of theoretical analysis. It shows that the mathematical model, the simulation method and the program in this thesis are correct.