Current　differential　protection　based　on　Kirchhoff＇s　law　has　sensitivity　problem　due　to　the　long-distance　DC　transmission　lines　with　distributed　parameter　effect.　In　this　paper,　the　charge　of　the　DC　transmission　line　under　the　situations　of　external　and　internal　faults　is　analyzed.　The　feature　is　that　the　charge　will　be　injected　into　the　fault　point　from　both　sides　of　the　DC　line　when　the　internal　faults　occur,　and　if　the　external　faults　occur,　the　distributed　capacitance　of　the　DC　line　will　be　discharged.　Then　a　pilot　protection　principle　based　on　charge　balance　is　proposed　using　the　polarity　of　the　sum　of　the　charge　in　a　time　period.　The　proposed　principle　is　simple　and　independent　of　the　parameters　of　the　DC　line,　and　it　theoretically　suits　for　both　transient　process　and　steady-state　process.　Finally,　an　MMC-HVDC　transmission　system　is　built　in　PSCAD/EMTDC,　and　the　simulation　results　show　that　the　method　is　reliable　and　not　affected　by　fault　resistance.　©　2019　IEEE.