A　multi-objective　optimal　method　of　monitors　for　voltage　sag　location　under　observability　constraint　is　proposed.　It　uses　positive　sequence　voltage　variation　as　sag　pattern　and　defines　the　distance　of　pattern　to　find　the　most　likely　fault　location.　Then　the　uncertainty　region　index　is　defined　to　measure　the　locating　accuracy　of　the　measuring　set.　Finally　the　multi-objective　monitoring　optimization　model　is　established　which　takes　the　observability　of　voltage　sag　as　the　constraint　condition　and　takes　the　minimum　number　of　monitoring　points　and　the　minimum　uncertainty　index　as　the　objective　function.　The　multi-objective　discrete　particle　swarm　optimization　algorithm　is　used　to　get　the　non-inferior　solution　set.　The　feasibility　and　validity　of　the　method　are　verified　by　IEEE　39　and　IEEE　118　node　system.　©　2019,　Electrical　Technology　Press　Co.　Ltd.　All　right　reserved.