Focusing　on　the　signal　extraction　for　dynamic　voltage　restorer　in　three-phase　three-wire　system,　an　advanced　detection　method　for　voltage　sag　based　on　unscented　Kalman　filter　is　presented　combining　with　mathematical　morphology　filter.　Nonlinear　state-space　model　for　fundamental　component　containing　steady-state　component　and　compensation　component　is　established　in　αβ　reference　frame.　Unscented　Kalman　filter　is　used　to　realize　the　model　and　mathematical　morphology　filter　is　designed　to　preprocess　the　random　noise　in　voltage　input　signal.　The　proposed　method　has　the　functions　of　steady-state　component　estimation,　compensation　component　extraction　and　decoupling　detection　for　positive/negative　sequence　component　amplitude　and　phase.　Especially,　the　proposed　method　has　advantages　of　fast　response　and　high　reliability.　Simulation　results　verify　the　correctness　and　effectiveness　of　the　proposed　method.