The real-time digital simulation RTDS has some shortcomings in the application of the large-scale HVAC and HVDC complex power system， such as the invisibility and the offline characteristic. In order to improve the application of RTDS， a graphical online simulation system based on RTDS is discussed and designed. The online power flow data from the energy management system（EMS） is initiated on RTDS as the initial state for real-time simulation by the state estimation program. As a result of state estimation， the initial power flow can be sent to flush the RTDS with a DNP communication protocol. The ground state data of the RTDS can thus be much closer to the actual power system. For the purpose of convenient application a graphics platform is developed and， via digital communication， the RTDS simulation results can be dynamically displayed and operated with the power network wiring diagram on the same interface. This system has been deployed in China Southern grid.