Voltage　sag　causes　abnormal　operation　of　the　sensitive　load.　Locating　sag　source　is　the　basis　of　troubleshooting　and　improving　power　quality.　The　methods　employed　to　locate　the　source　of　voltage　sags　rely　on　a　physical　quantity　that　is　sensitive　to　the　number　of　measurements　and　dependent　upon　the　voltage　sag　source.　A　vague-set-based　sag　source　location　method　is　proposed.　Firstly,　the　positive　sequence　set　of　the　node　voltage　is　calculated　in　different　faulty　lines　and　points,　and　the　positive　sequence　components　are　combined　with　the　Gauss　membership　function　of　the　monitoring　quantity　to　calculate　the　support　degree.　Secondly,　the　hesitancy　degree　and　opposing　degree　are　computed　and　the　vague　values　of　the　faulty　line　can　be　obtained　according　to　the　support　degree,　hesitancy　degree　and　opposing　degree.　In　addition,　the　similarity　between　the　vague　value　of　each　line　and　the　reference　value　[1,　1]　is　calculated.　Besides,　the　sag　source　positioning　index　of　each　line　is　calculated　by　the　largest　similarity　value　among　various　line　faults.　Finally,　taking　the　IEEE　14　and　IEEE　118　bus　systems　as　examples,　eight　types　of　similarity　definitions　are　used　to　verify　the　accuracy　of　the　method.　The　result　illustrates　that　the　proposed　method　has　a　good　performance　in　locating　the　source　of　voltage　sag　with　fewer　measurements　and　hypo-sensitivity　of　the　voltage　sag　source.　©　2023　IEEE.