With the advancement of the global energy interconnection strategy,many domestic scientific institutions have already done researches on the series multi-terminal HVDC systems applied into the cross-sea interconnection,LCC and VSC hybrid transmission and other area.However,the inherent characteristic that series converter can't be grounded in normal operation determines its reliability should be improved higher.Therefore,this paper analyzes the requirement of electrode topology for series multi-terminal HVDC transmission system in different operating modes and proposes an electrode topology of series multi-terminal HVDC system and its control method.At last,it is concluded that the electrode topology can effectively avoid the overall outage of the series multi-terminal HVDC system in the case of any equipment overhaul.