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.