A new detection method for voltage sag based on space vector is presented in this paper. Firstly, the equation of the space vector’s trajectory is calculated by applying Clark Transformation and fast ellipse reconstruction method to the voltage signals of three different times. Based on the space vector trajectory, voltage sag is detected and the fault phase was identified; at the same time, the phase angle jump can be calculated from the ellipse’s parameters. Then shifted the original signal data according to the phase angle jump, the depth of voltage sag could be calculated rapidly. The simulation results verify the effectiveness of this method.