Extruded insulated DC cable has been widely used in VSC-HVDC transmission systems. The reliability of DC cable system is challenged by the transient over-voltage caused by switching operation, internal fault and lightning stroke. Based on Nan’ao demonstration project, a simulation model of HVDC power cable system is established with PSCAD/EMTDC. Different types of faults including AC single-phase grounding fault, two-phase short circuit fault and arm reactor grounding fault are set up to study the transient characteristics of the DC cable system. Research results show that the maximum overvoltage peak at the DC cable terminal can run up to about 2 p.u.;and the over-voltage at DC cable terminal is affected by the fault resistance values.