Multi-terminal line commuter converter-high voltage direct current （LCC-HVDC） transmission can realize power supply from multiple energy bases or power receiving in multiple load centers，thus it is a flexible and economical technical scheme. For multi-terminal HVDC，online entry and exit of the third converter station is the key strategy to realize flexible switching of operation modes. Based on the technical roadmap of high speed switch，the online entry and exit strategy of the third converter station in multi-terminal LCC-HVDC system is studied，the advantages and disadvantages of strategies for different strategies are compared and analyzed from the perspectives of the performance of high-speed parellel switch，the impacts on equipment，and the power loss of AC system. In order to reduce the operation overstress of the equipment and create good conditions for the opening and closing of high speed switch，this paper proposes a strategy for online entry and exit of the third converter station in multi-terminal LCC-HVDC. Finally，the feasibility of the proposed strategy is verified by the field test results.