When a monopolar grounding fault occurs on DC line in Nan’ao multi-terminal VSC-HVDC project, it is necessary to ensure that the mechanical DC circuit breakers (DCCBs) can isolate the fault effectively and the non-fault stations can keep normal operation. Firstly the basic principle of the mechanical DCCBs recently installed in Nan’ao is introduced. Secondly, the line fault characteristics, the existing control and protection strategies, and the tripping characteristics of DCCBs are analyzed. Subsequently, a DC line differential protection with voltage criterion is put forward to locate the fault line quickly, and a method of switching setting values of protections in non-fault stations is presented to keep the normal operation of non-fault stations. Finally, the results of simulation in PSCAD/EMTDC and artificial short circuit test in Nan’ao project verify the feasibility of the strategy.