Water-supply　system　is　one　of　the　important　lifeline　systems,　which　plays　an　important　role　on　the　post-earthquake　rescuing,　reconstructing　and　rehabilitating.　The　paper　presents　a　new　modified　methods　to　assess　the　seismic　performance　of　water-supply　system　considering　interactions　between　water-electric　systems　during　earthquake.　The　interaction　between　water　supply　and　electric　system　are　estimated　based　on　graph　theory　(GT)　and　network　analysis　(NA).　With　GT　and　NA,　Both　of　water　supply　and　electric　systems　are　simulated　as　nodes　and　links,　and　the　nodes　are　farther　divided　into　simply　nodes　and　complicated　nodes,　the　former　represents　these　nodes　that　their　performance　will　not　be　affected　by　the　performance　of　electric　system,　the　latter　stand　for　these　nodes　that　their　perfomance　depends　on　the　other　elements　in　electric-power　systems.　The　interactions　between　water　supply　and　electric　systems　are　considered　in　complicated　nodes　through　the　definition　of　several　parameters　based　on　the　statistic　analysis.　The　links　are　applied　to　describe　the　pipeline　element　and　electric　transmitting　elements,　and　their　performance　levels　are　estimated　considering　the　effect　of　the　fault　movement,　ground　displacement,　and　peak　ground　acceleration.　Based　on　the　reliability　of　nodes　and　links,　the　water-supply　systemic　reliability　is　calculated　by　use　of　GT　and　NA.　Lastly,　compared　with　seismic　performance　of　water　supply　in　one　actual　earthquake　in　China,　one　illustration　is　given　to　test　the　reliability　of　the　proposed　method　in　this　paper.