The　problems　of　continuous　rise　in　global　power　consumption,　gradual　increase　in　the　peak-valley　difference　of　power　systems,　and　continuous　expansion　of　renewable　energy　penetration　have　caused　the　power　system　regulation　to　shift　from　the　traditional　supply　side　to　demand　side.　Buildings　have　huge　power　consumption　and　great　energy　flexibility,　making　them　one　of　the　most　important　units　on　the　demand　side　of　a　power　system.　This　study　performed　a　detailed　review　of　183　literatures　on　flexible　energy　resources.　The　review　has　been　carried　out　from　the　supply　and　demand　side　and　demand　response　strategy　side.　Based　on　the　results　of　the　analysis　and　discussion,　we　proposed　seven　general　quantitative　models　of　flexibility　for　commonly　used　flexible　resources,　which　lays　the　foundation　for　the　flexibility　evaluation　of　the　building　electric　energy　flexible　system　(BEEFS)　and　implementation　of　demand　response　strategies.　In　addition,　the　optimization　techniques　commonly　used　in　demand　response　strategies　are　analyzed　to　achieve　different　goals.　The　results　show　that　the　combination　of　these　strategies　and　optimization　techniques　can　achieve　the　goals　of　reducing　operating　costs　(OCs)　by　4%-42%,　reducing　peak　power　(PP)　by　15%-96%,　and　improving　renewable　energy　consumption　rate　(RECR)　by　6%-70%.　This　study　constructs　an　overall　framework　for　the　flexibility　quantification　of　building　electric　energy　systems,　which　can　be　further　expanded　in　the　future.　In　the　process　of　interaction　between　the　building　and　the　grid,　appropriate　demand　response　strategies　and　optimization　techniques　need　to　be　selected　according　to　the　types　and　characteristics　of　flexible　resources　to　achieve　the　goals　in　the　demand　response　process.