From the simulation test of Lingbao back-to-back DC transmission expanding project， the tap changer control of two-terminal DC transmission system and back-to-back DC transmission system， which have been put into operation in China， is an-alyzed and compared. On this basis， a new tap changer control method for inverter transformer of back-to-back DC system is proposed. And the principle of the method is that the reference of ideal no-load DC voltage varying with the DC current，so as to keep the DC voltage constant and make the extinction angle as small as possible. The feasibility of this new tap changer control method is proved by the simulation test of Lingbao back-to-back DC transmission expanding project.