For the new type of connection mode of ultra high-voltage alternate current (UHVDC) which is hierarchically connected to 1000 kV layer and 500 kV layer AC grid, the traditional UHVDC control system is no longer applicable. After analyzing the characteristics of this kind of connection mode, the control system for UHVDC hierarchical connection mode is studied. The biggest problem caused by series valve groups in the same pole being connected to different AC grid is valve groups voltage imbalance. In order to solve the problem of real-time valve voltage balancing from the control function, according to a DC voltage measuring point between the series valve groups, a new voltage of series valve balance control strategy is developd. Finally, a case of UHVDC transmission using RTDS validates the whole DC control system, provides a complete solution of control strategy for the UHVDC hierarchical connection project to be implemented in 2015.