In the real-time simulation, the converter transformer containing a large number of switching devices has the problem of insufficient storage capacity and computing power of the simulation hardware, which causes the simulation to be overloaded and seriously affects the development of the HVDC simulation work. In response to this problem, the ideal transformer model interface algorithm is used to design a converter transformer that implements on-load voltage regulation. Taking the CIGRE standard HVDC system test model as an example, offline and real-time simulations are performed on Simulink and RT-LAB platforms respectively. The steady-state and transient results of the ITM-type converter transformer without tap switch and the converter transformer with tap switch are consistent. Finally, it is concluded that the method of realizing the converter transformer based on the ideal transformer model interface algorithm can be applied to the construction of the real-time simulation test platform of DC transmission.