Smoothing　reactors,　together　with　the　DC　filter　banks　installed　at　both　ends　of　the　HVDC　transmission　line　constitute　the　＇natural　boundary＇,　which　leads　to　significant　difference　in　attenuation　characteristic　of　different　frequency　components　of　fault　transient　signals　detected　by　protection.　By　means　of　Hilbert-Huang　Transform,　which　is　a　kind　of　nonstationary　transient　signal　processing　method,　the　energy　of　high　frequency　components(2000Hz-10kHz)　is　extracted　based　on　the　attenuation　characteristic　of　the　line　boundary,　then　a　novel　algorithm　for　the　non-unit　transient-based　protection　is　acquired.　Moreover,　a　protection　threshold　that　is　adapt　to　various　lengths　of　transmission　line　is　obtained　by　extensive　simulation　work.　Experimental　results　indicate　that　the　new　criteria　can　identify　any　internal　fault　on　the　whole　line　and　clear　it　rapidly　and　credibly,　what＇s　more　important,　the　accuracy　and　reliability　of　the　criteria　will　not　be　influenced　by　the　length　of　the　transmission　line.　©2010　IEEE.