When　one　line　of　double　circuits　on　same　pole　or　in　parallel　has　fault,　the　healthy　line　contains　the　information　of　other　terminal,　which　can　be　used　to　locate　the　fault.　A　fault　locating　algorithm　is　proposed　for　it.　The　distributed　parameter　models　of　positive,　zero　and　negative　sequence　components　and　transmission　line　equations　are　adopted,　and　the　distance　of　fault　point　from　the　measuring　point　is　calculated　according　to　the　fault　information　got　at　one　line　terminal　and　based　on　the　pure　resistance　feature　of　actual　fault　point.　With　eigen-modulus　analysis,　the　mutual　admittance　and　impedance　of　parallel　lines　in　zero-sequence　network　are　de-coupled.　The　accuracy　of　algorithm　is　not　affected　by　the　distributed　capacitance　and　the　system　impedance　diversification　of　other　terminal.　The　accurate　calculation　shows　that　the　influence　of　phase　arrangement　on　locating　accuracy　can　be　corrected　by　using　error　correction　curve　or　table.